Values in MIME headers which are encoded using RFC2231 are not accepted. As the only header value which isn't ASCII (and thus might need to be encoded) is in ApiUpload this currently only accepts uploads with non-ASCII chars in the filename.
Another interpretation of this bug is also, why there is 'filename' in the header of the chunk/file entry of the MIME request as the server ignores it apparently. But that would just mask the underlying issue that there is no way to get Unicode data in the header values to the server (except by just using the encoding of the server but afaics is that not MIME compliant and Python 3's library doesn't support that).
Version: unspecified
Severity: normal
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=73661