Hi, Kendall - be very careful when you open e-mail attachments - and do your best not to send them yourself, except when you really need to. When I get an e-mail with an attachment, I often save it into a folder I labelled "hold for scan," and then I scan it with an up-to-date virus checker before I do anything else with it.

Once you've scanned for viruses, looke at the three-letter extension at the end of the file name, and it will tell you what kind of file it is. Click here for a guide hyberbolically called "Every File Format in the World." I can't imaging that it could cover every file format, but it should help you identify most of them and tell you how to open files of various formats.
-Joe Offer-