Find a way to regenerate empty thumbnails

[oparoz]

Maybe it's just on my test setup, but I can end up with blank thumbnails, probably because an error message was converted to a picture or something similar.

It would be nice to be able to find a way to re-generate just those.

We could do just one round trip if the preview was cached and its size is less than 500 bytes maybe.
In the future, if thumbnails feature a menu button, here could be an entry to regenerate just those thumbnails #39

[oparoz]

I don't think we can reliably check if a thumbnail is empty. If a thumbnail is not what it's supposed to be, an admin can regenerate the thumbnails for the folder.