The REST server uses the Content-Type header of the POST request to determine the content type of the stored entry. But with Content-Type: multipart/form-data, each parameter can have its own content type (and multipart/form-data is definitely not any parameter's content type).

This seems to be the way to do get the content type of each parameter: http://www.mkyong.com/webservices/jax-rs/file-upload-example-in-resteasy/