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.

Unit tests pass.
Manually tested with the new mimetypes, ensuring that thumbnails and

reviews worked correctly.
Verified the fix with a customer.