Support file upload with application/x-www-form-urlencoded

[fredbi]

Currently, the client can use a File reader to upload files with Content-Type multipart/form-data.

However, the swagger 2.0 spec states that files could also be uploaded as application/x-www-form-urlencoded.
This is not the recommended way, because this does not support streaming. However, it is formally supported by the spec...

See also go-swagger/go-swagger#2773