What are you talking about? MD5 is an algorithm used for file verification purposes. An MD5 file is just a file type that includes the name of the file you want to check and next to it goes the checksum, in this case an MD5 checksum.
If you downloaded an MD5 file from redump, expecting to get an ISO, you're simply so wrong. Redump simply offers a way to make sure that your self-obtained rip (by whatever method you decided to do so) is the same as the one that has redump considered correct.