R1:01862b8f23ac

Detect the MIME type of large files by examining the first chunk

Summary:
Fixes T11242. See that task for detailed discussion.

Previously, it didn't particularly matter that we don't MIME detect chunked files since they were all just big blobs of junk (PSDs, zips/tarballs, whatever) that we handled uniformly.

However, videos are large and the MIME type also matters.

- Detect the overall mime type by detecitng the MIME type of the first chunk. This appears to work properly, at least for video.