R1:17fc447503af

Don't compute MIME type of noninitial chunks from `diffusion.filecontentquery`

Summary:
Ref T12857. This is generally fairly fuzzy for now, but here's something concrete: when we build a large file with `diffusion.filecontentquery`, we compute the MIME type of all chunks, not just the initial chunk.

Instead, pass a dummy MIME type to non-initial chunks so we don't try to compute them. This mirrors logic elsewhere, in `file.uploadchunk`. This should perhaps be centralized at some point, but it's a bit…