I've got to recover some e-mails from MailArchiva, which unfortunately only gives out .mim files... which are .eml files if I manually change the name. But I'm going to have 300 of these, and I don't really want to do this by hand.
Select the files, click the Rename icon on the toolbar and put *.mim as the old name and *.eml as the new name.
(If the Advanced Rename dialog opens, make sure the Type is set to Wildcards. If the Simple one opens then you're good to go.)