You mean when you try a similar pattern in your own code?
If yes then it depends also how the call is made on the client-side. We rely on DropzoneJS so it’s all being taken care of. Peaking at their code I can see that you would need the MIME type to be "multipart/form-data" (see here for instance, or here an entire fallback form that it ships with).
Did you have any chance to investigate (debug) the possible bug further?