Allow renaming multiple files without number if extensions are different
Normally, we only allow renaming multiple files if the new file name contains a contiguous sequence of '#' placeholders, which are then replaced by numbers.
However, if all extensions are different, we can also rename the files without such a placeholder because the original extension is preserved when renaming.
This had been possible some time ago already. That this "accidental feature" was lost was a side effect of the fix for bug 318942.