Eli Zaretskii <eliz@gnu.org> writes:

[...]

>> So your idea is to keep `image-dired--get-create-thumbnail-file' as is
>> and filter its input in `image-dired-display-thumbs'?  But then we won't
>> get any message or error that something was not an image (this could go
>> unnoticed as well ;-)
>
> I don't necessarily see a reason to alert the user in this case, but
> if you think we had better done that, we could show a message when we
> find a non-image file in the list of the selected ones.

Ok.  Here is a new version of the patch that address those issues.  I
choose to not alert the user.