This adds support for various alternative JSON mimetypes and YAML
mimetypes, ensuring that thumbnails display content correctly and that
they can be reviewed as text files when using an `application/`
mimetype.

The text-based review UI uses the text-based mimetype as the basis for
the list of supported mimetypes, so these won't get out of sync.

Testing Done:
Unit tests pass.

Manually tested with the new mimetypes, ensuring that thumbnails and
reviews worked correctly.

Verified the fix with a customer.

Reviewed at https://reviews.reviewboard.org/r/12730/