Problem: Multipart uploads where the first part was a form field and a later part contained the file used the first part's Content-Type for the file metadata.
Root cause: After finding a later part with a filename, parseUpload copied data and MD5 from part2 but read Content-Type from the original part variable.
Fix: Read Content-Type from the discovered file part.
Reproduction: go test ./weed/storage/needle -run TestParseUploadUsesDiscoveredFilePartContentType -count=1 failed before the fix because the parsed MIME type was text/plain instead of application/x-seaweed-test.
Validation: go test ./weed/storage/needle -run TestParseUploadUsesDiscoveredFilePartContentType -count=1; go test ./weed/storage/needle -count=1; git diff --check; git diff --cached --check