Display correct file type on download button

Works, but awkward - the extension name changes a little later than the button
text itself. This is hard to avoid since we don't know beforehand
what the thumbnail type is - we have to wait for the API request.
(We can't do thumbnail URL guessing here since we cannot catch the 404 error.)

In general the whole API handling here is not so good, with a separate API
request going out when we should just get all size options in a single request,
when the user opens the reuse panel.

Change-Id: I502b7cb4e99d8af348d7d1967eb8343ec0f926fe
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/471
Merge "Display correct file type on download button"