Your Internet Consultant - The FAQs of Life Online
If you can see the filename (that is, if the mystery file is sitting in your hard disk or included in the subject of a message), BtoA-encoded filenames usually end in the extension .btoa or the more verbose .MBin.ascii. If you can't see the filename, look at the first line of the message. The first line of a BtoA file starts with something similar to xbtoa Begin.