Non-plaintext document thumbnailer

[DangerOnTheRanger]

A nice feature to have would be thumbnailing text documents in formats besides plaintext, ideally using a proper rendering as the thumbnail. Latex and PDF would be reasonably simple, ODT and dockx might be harder/impossible to do cleanly, but Latex and PDF would still be a good start, I think.

At some point viewing documents in-browser without needing to download them would be nice (same goes for other non-browser-native formats), but that's better left to a separate ticket.

[DonaldTsang]

Also EPUB, MOBI and DJVU should be considered