multipart/form-data doesn't send cookies

I use a session cookie to authenticate the API client, but if the client tries to upload a file to the server, the server receives no cookies (I've debugged server-side to be certain) and so it denies access.
**Everyhting works fine if the request is not `multipart/form-data`.**
The file does get sent successfully, though.

No file or test is identified in the issue. Start by reproducing a multipart/form-data upload using a session cookie, then trace the request path to determine where cookies stop being sent. Done means the server receives the session cookie for multipart uploads while the file continues to upload successfully.

Written by the indexing model from the issue text.