I suggest to add a function to rename by pasting from clipboard.
My idea is as follows:
1) copy a filename by pressing [Control-C] outside Multi-Commander,
2) highlight the file to be renamed inside Multi-Commander,
3) click the icon to rename by pasting from clipboard
4) a dialogue box appears asking for confirmation. You have the chance to modify the filename, to change the extension. Press [OK] to rename or press [Cancel] to abort.
Thanks.