A workaround could be that you send the file as Base64 encoded string instead of multipart request.
Hi Frederic,
I also have issues with multipart/form-data when sending a FileDocument in Mendix 8.7.
When the server is receiving the Content-Type header, it is not able to parse the boundary. It looks like Mendix puts an extra line break in the boundary and therefore malform the header.
I reported an issue with Mendix two weeks ago (#98364), there is no solution yet.
EDIT: Mendix reported that they’re fixing this in 8.9.
Best,
Nick