Show a thumbnail of all representable Active Storage files + images

[wvengen]

Currently, only images have a preview when using Active Storage attachments. Preview of videos (and potentially other file formats) are supported, however, but one needs to use representable instead of only previewable. This PR switches to using representable.

Tested with movie previews and images on multiple upload.

Note that we still need to look at the image mime-type, to show e.g. SVG images (which are not representable, but can still be shown directly - great that there is an automated test for this).

See also:

• https://edgeguides.rubyonrails.org/active_storage_overview.html
• https://edgeapi.rubyonrails.org/classes/ActiveStorage/Blob/Representable.html

[wvengen]

It's not clear to me why one test fails (jruby-9.3, rails_6.1, active_record, mysql2, sprockets) and all the others succeed. It doesn't look like it would be related to the changes in this PR (timeout errors). (after a rebase all tests succeeded)

[coveralls]

coverage: 95.996% (+0.1%) from 95.9%
when pulling 1246376 on wvengen:activestorage-representable
into af7414a on railsadminteam:master.