Show (and maybe enforce) allowed formats on image upload form

[karencfv]

When uploading an image it would be nice to have a small description of which image formats are accepted. Perhaps also a link to https://docs.oxide.computer/guides/creating-and-sharing-images.

Additionally, it's difficult to know which block size to choose. If 512 is the default, perhaps write "default" next to it?

[david-crespo]

Definitely. We've discussed all of those at various times. Regarding the formats, I agree we should say something right in the UI, and I feel the full doc could be clearer too.

#1669 (review)

[david-crespo]

Per oxidecomputer/omicron#3559 (comment), probably also worth doing a check on the filename (or inferred mime type) and rejecting extensions we know won't work.

[david-crespo]

Fixed by #2217 and de7e443 (link to images doc at the top of the form).