application/vnd.flow.v2+json

[1] API Conventions (2017-09-02 07:16:24 +09:00) <https://developer.getflow.com/conventions/>

For POST and PUT requests, you will need to specify a Content-Type header with your request to ensure the JSON you send is parsed correctly.
Simply include the Content-Type header:
Content-Type: application/vnd.flow.v2+json

[2] API Conventions (2017-09-02 07:16:24 +09:00) <https://developer.getflow.com/conventions/>

Specifying version via the Accept header
The most transparent way is to ensure you set the Accept HTTP header to our custom content-type declaration, which includes a version string. For v2, should look like:
Accept: application/vnd.flow.v2+json