Status of this Memo

This memo defines an Experimental Protocol for the Internet community. This memo does not specify an Internet standard of any kind. Discussion and suggestions for improvement are requested. Distribution of this memo is unlimited.

1. Abstract

Currently, HTML forms allow the producer of the form to request information from the user reading the form. These forms have proven useful in a wide variety of applications in which input from the user is necessary. However, this capability is limited because HTML forms don't provide a way to ask the user to submit files of data. Service providers who need to get files from the user have had to implement custom user applications. (Examples of these custom browsers have appeared on the www-talk mailing list.) Since file-upload is a feature that will benefit many applications, this proposes an extension to HTML to allow information providers to express file upload requests uniformly, and a MIME compatible representation for file upload responses. This also includes a description of a backward compatibility strategy that allows new servers to interact with the current HTML user agents.

The proposal is independent of which version of HTML it becomes a part.

2. HTML forms with file submission

The current HTML specification defines eight possible values for the attribute TYPE of an INPUT element: CHECKBOX, HIDDEN, IMAGE, PASSWORD, RADIO, RESET, SUBMIT, TEXT.

In addition, it defines the default ENCTYPE attribute of the FORM element using the POST METHOD to have the default value "application/x-www-form-urlencoded".

This proposal makes two changes to HTML:

• 1) Add a FILE option for the TYPE attribute of INPUT.
• 2) Allow an ACCEPT attribute for INPUT tag, which is a list of media types or type patterns allowed for the input.

この提案は、 HTML に2つの変更を加えます。

• INPUT の TYPE 属性に FILE 選択肢を加える。
• INPUT タグに ACCEPT 属性を加え、入力を認める媒体型又は型パターンの並びを指定できるようにする。

In addition, it defines a new MIME media type, multipart/form-data, and specifies the behavior of HTML user agents when interpreting a form with ENCTYPE="multipart/form-data" and/or <INPUT type="file"> tags.

加えて、この提案は multipart/form-data という新しい MIME 媒体型を定義し、 ENCTYPE="multipart/form-data" 及び/又は <INPUT type="file"> タグのあるフォームを解釈する時の HTML 利用者エージェントの動作を規定します。

These changes might be considered independently, but are all necessary for reasonable file upload.

The author of an HTML form who wants to request one or more files from a user would write (for example):

    <FORM ENCTYPE="multipart/form-data" ACTION="_URL_" METHOD=POST>
    File to process: <INPUT NAME="userfile1" TYPE="file">
    <INPUT TYPE="submit" VALUE="Send File">
    </FORM>

The change to the HTML DTD is to add one item to the entity "InputType". In addition, it is proposed that the INPUT tag have an ACCEPT attribute, which is a list of comma-separated media types.

  ... (other elements) ...
  <!ENTITY % InputType "(TEXT | PASSWORD | CHECKBOX |
                         RADIO | SUBMIT | RESET |
                         IMAGE | HIDDEN | FILE )">
  <!ELEMENT INPUT - 0 EMPTY>
  <!ATTLIST INPUT
          TYPE %InputType TEXT
          NAME CDATA #IMPLIED  -- required for all but submit and reset
          VALUE CDATA #IMPLIED
          SRC %URI #IMPLIED  -- for image inputs --
          CHECKED (CHECKED) #IMPLIED
          SIZE CDATA #IMPLIED  --like NUMBERS,
                                  but delimited with comma, not space
          MAXLENGTH NUMBER #IMPLIED
          ALIGN (top|middle|bottom) #IMPLIED
          ACCEPT CDATA #IMPLIED --list of content types
          >
  ... (other elements) ...

3. Suggested implementation

While user agents that interpret HTML have wide leeway to choose the most appropriate mechanism for their context, this section suggests how one class of user agent, WWW browsers, might implement file upload.

3.3 use of multipart/form-data

The definition of multipart/form-data is included in section 7. A boundary is selected that does not occur in any of the data. (This selection is sometimes done probabilisticly.) Each field of the form is sent, in the order in which it occurs in the form, as a part of the multipart stream. Each part identifies the INPUT name within the original HTML form. Each part should be labelled with an appropriate content-type if the media type is known (e.g., inferred from the file extension or operating system typing information) or as application/octet-stream.

multipart/form-data の定義は7章に含めています。 境界 (boundary) はデータのどこにも現れないものから選びます。 (この選択は時たま確率論的なものとなります。) フォームの各欄は、フォーム中に出現した順序で、 複数部分流の一つの部分として送信します。 各欄は元の HTML フォーム中の INPUT 名で識別します。各部分は、媒体型がわかっているとき (例えばファイルの拡張子やオペレーティング・システムの型情報から推測できるとき) には、適当な内容型で、そうでないときには application/octet-stream で札付けするべきです。

If multiple files are selected, they should be transferred together using the multipart/mixed format.

複数のファイルが選択されたときには、各ファイルは一緒に multipart/mixed 書式を使って転送するべきです。

While the HTTP protocol can transport arbitrary BINARY data, the default for mail transport (e.g., if the ACTION is a "mailto:" URL) is the 7BIT encoding. The value supplied for a part may need to be encoded and the "content-transfer-encoding" header supplied if the value does not conform to the default encoding. [See section 5 of RFC 1521 for more details.]

HTTP プロトコルは任意の BINARY データを転送できますが、 メイル転送 (例えば、 ACTION が mailto: URL のとき) の既定値は 7BIT 符号化です。 部分に供給される値は、その値が既定符号化に適合しなければ、 符号化して content-transfer-encoding 頭をつける必要があるかもしれません。 (詳細については RFC 1521 の5章を参照。)

The original local file name may be supplied as well, either as a 'filename' parameter either of the 'content-disposition: form-data' header or in the case of multiple files in a 'content-disposition: file' header of the subpart. The client application should make best effort to supply the file name; if the file name of the client's operating system is not in US-ASCII, the file name might be approximated or encoded using the method of RFC 1522. This is a convenience for those cases where, for example, the uploaded files might contain references to each other, e.g., a TeX file and its .sty auxiliary style description.

元の局所ファイル名は、 content-disposition: form-data 頭の filename 引数として、又は複数ファイル群の場合には、 その小部分の content-disposition: file 頭において、同様に供給してもかまいません。 クライアント応用はファイル名を供給する最善の努力をするべきです。 送信者のオペレーティング・システムのファイル名が US-ASCII でないなら、ファイル名は近似するか、 RFC1522 の方法を使って符号化しても構いません。 フォームで供給するファイル群が相互の参照を含んでいる場合、 例えば TeX ファイルとその .sty 追加スタイル記述のような場合には便利です。

On the server end, the ACTION might point to a HTTP URL that implements the forms action via CGI. In such a case, the CGI program would note that the content-type is multipart/form-data, parse the various fields (checking for validity, writing the file data to local files for subsequent processing, etc.).

サーバー側では、 ACTION はフォーム動作を CGI を介して実装している HTTP URL を指しているかもしれません。 そのような場合には、 CGI プログラムは content-type が multipart/form-data であることに気づき、 種々の欄を解析する (妥当性を確認し、ファイル・データを以後の処理のために局所ファイルに書き込むとか。) ことでしょう。

4. Backward compatibility issues

While not necessary for successful adoption of an enhancement to the current WWW form mechanism, it is useful to also plan for a migration strategy: users with older browsers can still participate in file upload dialogs, using a helper application. Most current web browers, when given <INPUT TYPE=FILE>, will treat it as <INPUT TYPE=TEXT> and give the user a text box. The user can type in a file name into this text box. In addition, current browsers seem to ignore the ENCTYPE parameter in the <FORM> element, and always transmit the data as application/x-www-form-urlencoded.

Thus, the server CGI might be written in a way that would note that the form data returned had content-type application/x-www-form-urlencoded instead of multipart/form-data, and know that the user was using a browser that didn't implement file upload.

In this case, rather than replying with a "text/html" response, the CGI on the server could instead send back a data stream that a helper application might process instead; this would be a data stream of type "application/x-please-send-files", which contains:

この場合、 text/html 応用を返信するよりは、 サーバーの CGI は補助応用が代わりに処理するかもしれないデータ流を代わりに送ることができます。 これは型 application/x-please-send-files のデータ流とすると、次のものを含むでしょう。

• * The (fully qualified) URL to which the actual form data should be posted (terminated with CRLF)
• * The list of field names that were supposed to be file contents (space separated, terminated with CRLF)
• * The entire original application/x-www-form-urlencoded form data as originally sent from client to server.

• 実際のフォーム・データを投函するべき (完全修飾) URL (CRLF で終端)
• ファイル内容を期待している欄名の並び (間隔区切、 CRLF で終端)
• クライアントからサーバーに元々送られてきた元の application/x-www-form-urlencoded フォーム・データ全体。

In this case, the browser needs to be configured to process application/x-please-send-files to launch a helper application.

The helper would read the form data, note which fields contained 'local file names' that needed to be replaced with their data content, might itself prompt the user for changing or adding to the list of files available, and then repackage the data & file contents in multipart/form-data for retransmission back to the server.

The helper would generate the kind of data that a 'new' browser should actually have sent in the first place, with the intention that the URL to which it is sent corresponds to the original ACTION URL. The point of this is that the server can use the *same* CGI to implement the mechanism for dealing with both old and new browsers.

The helper need not display the form data, but *should* ensure that the user actually be prompted about the suitability of sending the files requested (this is to avoid a security problem with malicious servers that ask for files that weren't actually promised by the user.) It would be useful if the status of the transfer of the files involved could be displayed.

5. Other considerations

5.2 Deferred file transmission

In some situations, it might be advisable to have the server validate various elements of the form data (user name, account, etc.) before actually preparing to receive the data. However, after some consideration, it seemed best to require that servers that wish to do this should implement this as a series of forms, where some of the data elements that were previously validated might be sent back to the client as 'hidden' fields, or by arranging the form so that the elements that need validation occur first. This puts the onus of maintaining the state of a transaction only on those servers that wish to build a complex application, while allowing those cases that have simple input needs to be built simply.

The HTTP protocol may require a content-length for the overall transmission. Even if it were not to do so, HTTP clients are encouraged to supply content-length for overall file input so that a busy server could detect if the proposed file data is too large to be processed reasonably and just return an error code and close the connection without waiting to process all of the incoming data. Some current implementations of CGI require a content-length in all POST transactions.

If the INPUT tag includes the attribute MAXLENGTH, the user agent should consider its value to represent the maximum Content-Length (in bytes) which the server will accept for transferred files. In this way, servers can hint to the client how much space they have available for a file upload, before that upload takes place. It is important to note, however, that this is only a hint, and the actual requirements of the server may change between form creation and file submission.

In any case, a HTTP server may abort a file upload in the middle of the transaction if the file being received is too large.

5.3 Other choices for return transmission of binary data

Various people have suggested using new mime top-level type "aggregate", e.g., aggregate/mixed or a content-transfer-encoding of "packet" to express indeterminate-length binary data, rather than relying on the multipart-style boundaries. While we are not opposed to doing so, this would require additional design and standardization work to get acceptance of "aggregate". On the other hand, the 'multipart' mechanisms are well established, simple to implement on both the sending client and receiving server, and as efficient as other methods of dealing with multiple combinations of binary data.

色々な人が、新しい mime 最上位型 aggregate (修正) を使って例えば aggregate/mixed としたり、 不定長バイナリ・データの表現に複数部分様式の境界に依存するのではなく packet 内容転送符号化を使ったりすることを提案しています。 そうすることに反対はしませんが、 aggregate の追加の設計と承認のための標準化作業が必要になります。他方、 multipart 機構はよく確立しており、送信するクライアント及び受信するサーバーの両方を実装するのが簡単であり、 バイナリ・データの複数の組合せを処理する他の方式と同じくらい有効です。

5.5 Default content-type of field data

Many input fields in HTML are to be typed in. There has been some ambiguity as to how form data should be transmitted back to servers. Making the content-type of <INPUT> fields be text/plain clearly disambiguates that the client should properly encode the data before sending it back to the server with CRLFs.

HTML の多くの入力欄は型付けされます。 どうフォーム・データをサーバーに送り返すべきかには幾らかの曖昧性があります。 <INPUT> 欄の内容型を text/plain にすることによって、サーバーにこれを送る返す前に CRLF で適当に符号化するべきであることを明確に曖昧でなくできます。

6. Examples

Suppose the server supplies the following HTML:

     <FORM ACTION="http://server.dom/cgi/handle"
           ENCTYPE="multipart/form-data"
           METHOD=POST>
     What is your name? <INPUT TYPE=TEXT NAME=submitter>
     What files are you sending? <INPUT TYPE=FILE NAME=pics>
     </FORM>

and the user types "Joe Blow" in the name field, and selects a text file "file1.txt" for the answer to 'What files are you sending?'

The client might send back the following data:

        Content-type: multipart/form-data, boundary=AaB03x
        --AaB03x
        content-disposition: form-data; name="field1"
        Joe Blow
        --AaB03x
        content-disposition: form-data; name="pics"; filename="file1.txt"
        Content-Type: text/plain
         ... contents of file1.txt ...
        --AaB03x--

If the user also indicated an image file "file2.gif" for the answer to 'What files are you sending?', the client might client might (>>6) send back the following data:

        Content-type: multipart/form-data, boundary=AaB03x
        --AaB03x
        content-disposition: form-data; name="field1"
        Joe Blow
        --AaB03x
        content-disposition: form-data; name="pics"
        Content-type: multipart/mixed, boundary=BbC04y
        --BbC04y
        Content-disposition: attachment; filename="file1.txt"
        Content-Type: text/plain
        ... contents of file1.txt ...
        --BbC04y
        Content-disposition: attachment; filename="file2.gif"
        Content-type: image/gif
        Content-Transfer-Encoding: binary
          ...contents of file2.gif...
        --BbC04y--
        --AaB03x--

7. Registration of multipart/form-data

The media-type multipart/form-data follows the rules of all multipart MIME data streams as outlined in RFC 1521. It is intended for use in returning the data that comes about from filling out a form. In a form (in HTML, although other applications may also use forms), there are a series of fields to be supplied by the user who fills out the form. Each field has a name. Within a given form, the names are unique.

媒体型 multipart/form-data は、 RFC 1521 で概説されている全複数部分 (multipart) MIME データ流の規則に従います。 (HTML の) フォームには (他の応用もフォームを使うかもしれませんが)、 フォームに記入した利用者によって供給された欄 (field) の系列があります。各欄には名前があります。 あるフォームの中では、名前は唯一無二です。

multipart/form-data contains a series of parts. Each part is expected to contain a content-disposition header where the value is "form-data" and a name attribute specifies the field name within the form, e.g., 'content-disposition: form-data; name="xxxxx"', where xxxxx is the field name corresponding to that field. Field names originally in non-ASCII character sets may be encoded using the method outlined in RFC 1522.

multipart/form-data は部分 (part) の系列を含みます。 各部分は配置型が form-data である content-disposition (内容配置) 頭 (header) を含むことを想定します。例えば content-disposition: form-data; name="xxxxx" で、 xxxxx はその欄に対応する欄の名前です。 元々非 ASCII 文字集合で書かれていた欄名は、 name 引数の値の中では RFC 1522 で説明されている方式を使って符号化しても構いません。

As with all multipart MIME types, each part has an optional Content-Type which defaults to text/plain. If the contents of a file are returned via filling out a form, then the file input is identified as application/octet-stream or the appropriate media type, if known. If multiple files are to be returned as the result of a single form entry, they can be returned as multipart/mixed embedded within the multipart/form-data.

全ての複数部分 MIME 型について、各部分は任意選択の Content-Type があって、その既定値は text/plain になっています。 ファイルの内容がフォームの記入によって返される時には、 application/octet-stream 又は適切な媒体型が分かっていればそれで識別します。単一のフォーム項目の結果として複数のファイル群を返す時には、 multipart/form-data 中に埋め込まれた multipart/mixed として表現するべきです。

Each part may be encoded and the "content-transfer-encoding" header supplied if the value of that part does not conform to the default encoding.

各部分の値が既定の符号化に適合しないなら、 その部分は符号化して content-transfer-encoding 頭をつけても構いません。

File inputs may also identify the file name. The file name may be described using the 'filename' parameter of the "content-disposition" header. This is not required, but is strongly recommended in any case where the original filename is known. This is useful or necessary in many applications.

ファイル入力はファイル名をも識別するかもしれません。 ファイル名は content-disposition 頭の filename 引数を使って記述します。 これは必須ではありませんが、元のファイル名を知っている場合には強く推奨します。 これは多くの応用で有用或いは必要です。

8. Security Considerations

It is important that a user agent not send any file that the user has not explicitly asked to be sent. Thus, HTML interpreting agents are expected to confirm any default file names that might be suggested with <INPUT TYPE=file VALUE="yyyy">. Never have any hidden fields be able to specify any file.

This proposal does not contain a mechanism for encryption of the data; this should be handled by whatever other mechanisms are in place for secure transmission of data, whether via secure HTTP, or by security provided by MOSS (described in RFC 1848).

Once the file is uploaded, it is up to the receiver to process and store the file appropriately.

9. Conclusion

The suggested implementation gives the client a lot of flexibility in the number and types of files it can send to the server, it gives the server control of the decision to accept the files, and it gives servers a chance to interact with browsers which do not support INPUT TYPE "file".

The change to the HTML DTD is very simple, but very powerful. It enables a much greater variety of services to be implemented via the World-Wide Web than is currently possible due to the lack of a file submission facility. This would be an extremely valuable addition to the capabilities of the World-Wide Web.

Authors' Addresses

   Larry Masinter
   Xerox Palo Alto Research Center
   3333 Coyote Hill Road
   Palo Alto, CA 94304

   Phone:  (415) 812-4365
   Fax:    (415) 812-4333
   EMail:   masinter@parc.xerox.com

   Ernesto Nebel
   XSoft, Xerox Corporation
   10875 Rancho Bernardo Road, Suite 200
   San Diego, CA 92127-2116

   Phone:  (619) 676-7817
   Fax:    (619) 676-7865
   EMail:   nebel@xsoft.sd.xerox.com

A. Media type registration for multipart/form-data

Media Type name

multipart

Media subtype name

form-data

Required parameters

none

Optional parameters

none

Encoding considerations

No additional considerations other than as for other multipart types.

Published specification

RFC 1867

References

[RFC 1521] MIME (Multipurpose Internet Mail Extensions) Part One:

           Mechanisms for Specifying and Describing the Format of
           Internet Message Bodies.  N. Borenstein & N. Freed.
           September 1993.

[RFC 1522] MIME (Multipurpose Internet Mail Extensions) Part Two:

           Message Header Extensions for Non-ASCII Text. K. Moore.
           September 1993.

[RFC 1806] Communicating Presentation Information in Internet

           Messages: The Content-Disposition Header. R. Troost & S.
           Dorner, June 1995.

ライセンス

RFCのライセンス

メモ

• [2] HTML 3.2 は file upload まわりの処理を RFC 1867 に丸投げしています。
• [3] RFC ERRATA <http://www.rfc-editor.org/errata.html> : 例中の , boundary は誤りで、 ; boundary が正解です。
• [4] >>3 どうでもいいけど MIME 系 RFC ってこの種の例文中の誤りが多いね。
• [5] ERRATA TO RFC 1867 <http://larry.masinter.net/file-upload-errata.txt> : 著者による errata。 >>3 の修正に加えて、6章の typo の修正も載っています。
• [6] IMPLEMENTATIONS OF WEB FILE UPLOAD <http://larry.masinter.net/file-upload-impl.txt> : 同じく著者による実装リスト。
• [7] 面白いことに、 >>5-6 で著者は「RFC 1867 は HTML 4 により廃止された」と認識していることがわかります。 (ちなみに RFC 2388 も RFC 2854 も著者は RFC 1867 と同じ彼。) 確かに HTML 4 の規定は RFC 1867 からそのまま引っ張ってきたようなものです。 (その仕事も彼が担当したのかもしれません。)

[8] RFC 1867 - Form-based File Upload in HTML (2007-01-14 06:54:18 +09:00 版) <http://tools.ietf.org/html/rfc1867>