X-Faceの形式を示す領域。Content-Type:領域風。

  x-face-type         = "X-Face-Type:" FWS x-face-type-content / obs-x-face-type
  obs-x-face-type     = "X-Face-Type" [FWS] ":" [CFWS] x-face-type-content
  x-face-type-content = (type / parameter) *(";" FWS parameter)
                      / obs-x-face-type-content
  obs-x-face-type-content = (type / parameter) *(";" [CFWS] parameter)
  type                = "RGB" / "GRAY" / <RFC 2045 token>
  parameter           = (animate-parameter
                       / depth-parameter
                       / geometry-parameter
                       / status-parameter        ; obsolete
                       / regular-parameter       ; for future extension
                       / extended-parameter)
  animate-parameter   = "animate=" 1*DIGIT "." 1*DIGIT
  depth-parameter     = "depth=" 1*DIGIT
  geometry-parameter  = "geometry=" 1*DIGIT "x" 1*DIGIT
  status-parameter    = "status=" <RFC 2045 value>
  regular-parameter   = <RFC 2231 regular-parameter>
  extended-parameter  = <RFC 2231 extended-parameter>

See X-Face.