It's the 21st century; do you know what's in your email box? There's a dirty little secret in cyberspace and it's that more people than not -- even people who should know better -- don't know how to handle a lot of the attachments they receive with their mail. If it doesn't say .doc, .gif, .jpg or .zip (on a PC) and .sit (on a Mac), they freak. Dealing with an .xls, .tga or .mp3 is easier than you think. So, the next time you get a mysterious attachment in your mailbox, stay calm, and consult the little DLT cheat sheet.

The Basics What most users don't seem to realize is that Mac and PC compression programs are now cross-platform. If you're on a PC and get an attachment with a .sit extension on the file name that means it's a file compressed using the popular Mac utility program called StuffIt. WinZip and other .zip compression/decompression programs for the PC can decompress these files, and as long as the files they contain are PC-friendly documents, you can open them as you would any other PC files. If you're using a Mac and get a .zip attachment, StuffIt (which comes with most Internet services, Web browsers, etc.) will open these compressed documents without a hitch.

Tip: If you're sending documents from one computer type to another, it's a good idea to add the appropriate three-letter file extension to the documents. If you send a Microsoft Word .doc file from a Mac to a PC without the .doc, the person on the other end may have trouble opening it. On the receiving end, if you have trouble opening a file by double-clicking on it, try opening up the application it was created in and then opening the document from there.

Multimedia Files Besides word processing documents, most of what you get in attachments are graphics and audio files. Most every graphic format (.gif, .jpg, .tga, .png, .bmp) can be opened (on either a Mac or PC) with an image-processing program such as Photoshop. Multimedia players, such as the Microsoft Media Player (which comes with both Mac and PC versions of Internet Explorer) can handle both video formats (.mpg, .avi, .qt, and .mov), as well as many popular audio formats (.mpg, mp3, .ram).

Tip: If you get a file attachment and you're not sure what it is, try opening it in your decompression program, Photoshop, MS Media Player and MS Word. One of these programs will likely do the trick.

Other Formats Here are a few other file formats you may encounter and what to do with them:

• Surf Safe
• Gadget Convergence