It may be better still to specify which characters are invalid...

And when auto-generated names turn out to be invalid, something's wrong. :) We should make sure the auto name is always valid or else clearly make it easy to fix.

(In reply to comment #1)

It may be better still to specify which characters are invalid...

That looks like another feature request, in the meanwhile matanya's suggestion could be implemented, but I'm not sure that "characters that are not allowed" is the best, given it's not only about characters but also sequences of characters/other regexes.

You now have no feedback. Instead your proposed title is automatically 'cleansed' and you have no feedback that your title got changed until after the real upload was executed.

Also not really ideal I think...

*** Bug 43009 has been marked as a duplicate of this bug. ***

Ok, the issue as originally reported is resolved -- the original filename is automatically converted to a suggested filename that removes the '[' and ']' chars, and you don't get a weird error message if you forget to customize the filename.

Resolving as fixed. If there remain concerns about what happens if you then type in a name that's not a valid title or file, please open a separate bug for that.