Support multipart media type by default for filefield/imagefield

[ychab]

See issue #386

With that fix, it means that we may always use Multipart media type instead of JSON parser... So not really sure if it is a good idea or not :/

A better alternative would be to have to select the content type in the UI of Swagger or Redoc instead....

[ychab]

Oh, I probably know why this is not possible and why it decide to switch on application/json media type by default if we don't have only form media type : this is because multipart/form-data doesn't allow nested parameter... !!
I just refresh the pull request in order to add some comments about this because this is really tricky and it could help to understand why (if this is the good reason anyway)

[axnsan12]

Yeah, your comments pretty much hit it on the head. The problem is both that there is no widely accepted standard for nested values in multipart/form-data, and that OpenAPI 2.0 enforces this separation via Parameters for form fields and Schemas for json body.

OpenAPI 3 is a real big step forward in this regard since it allows all request body types to be specified as a Schema, with a lot of knobs for media types. However building a lib to generate that is probably a nightmare.