Allow renaming multiple files without number if extensions are different

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.

BUG: 321234
FIXED-IN: 4.10.5
REVIEW: 111079