Open
Description
As seen on Fennec, some boxes just don't have mimetypes registered see:
pocmo/Fennec-DLC-Catalog@b6abf51
we should deal with this in kinto-attachement. https://github.com/Kinto/kinto-attachment/blob/master/scripts/upload.py#L61
maybe by raising an error ? another option is to guess it on the server-side in Kinto when it's not provided by the client maybe ?