Suffixes used on Windows computers - partial list:

Image Files
.tif    TIFF image file
.jpg    JPEG image file
.eps    Encapsulated Postscript file
.psd    Photoshop document
.gif    Compuserve GIF image file
.pct    Macintosh PICT image file
.png    Portable Network Graphics file
.pcx    PC Paintbrush bitmap image file
.mac    Macpaint bitmap file
.fpx    Kodak Flash Pix file
.bmp    Microsoft Bitmap image file
.wmf    Microsoft Windows metafile
.tga    Targa image file
.sct    Scitex bitmap files
.pxr    Pixar image file
.cdr    CorelDraw file
.fh5    Freehand version 5 file
.ai    Adobe Illustrator file

Multimedia Files
.mpg    MPEG compressed video
.mov    QuickTime movie -or-
.qt    QuickTime movie
.pic    QuickTime picture (I suspect that this is same as a PICT file)
.avi    Microsoft Video movie
.wav    Wave audio sound recordings
.mp3    sound file
.aif    sound file -or-
.aiff    sound file
.mid    MIDI synthesized sound
.cst    Macromedia Director cast files
.dir    Macromedia Director movie files - Note: this might be bad form in DOS land. DIR is reserved for directories (folders), and is a command that lists the contents of a folder.
.cxt    Macromedia Director protected cast files
.dxr    Macromedia Director protected movie files
.skl    Macromedia Director resources
.prf    Macromedia Director settings
.cct    Macromedia Shockwave Cast files
.dcr    Macromedia Shockwave movie
.spl    Shockwave Flash object -or-
.swf    Shockwave Flash object
.htm    Web document -or-
.html    Web document
.ra    Real Media file -or-
.ram    Real Media file -or-
.rm    Real Media file


Text Files
.txt    plain text file (generic text - no formatting)
.doc    Microsoft Word document
.wpd    WordPerfect document (WordPerfect doesn't need an extension, but it's good form)
.wri    Microsoft Write applet document
.rtf    Rich Text document (supported both Mac and Windows)

Page Layout Files
.qxd    QuarkXpress file
.pm5    PageMaker version 5 file
.pm6    PageMaker version 6 file
.p65    PageMaker version 6.5 file
.ind    InDesign version 1.5 file -or-
.indd    InDesign version 1.5 or later file

Fonts
.pfb    Postscript outline files
.pfm    Postscript font metric files
.mmm    Postscript Multiple Master font files
.ttf    TrueType font file
.otf    Open Type font file (new format with more characters, both Mac and Windows)

Other
.exe    programs
.dll    dynamic linked libraries for programs
.sys    operating system files
.scr    screen saver file
.drv    device drivers for operating system
.zip    ZIP compressed files
.pdf    Adobe Acrobat files
.pt    Kodak precision transform color correction file
.icc    color profile -or-
.icm    color profile (preferred)
.fp3    FileMaker Pro version 3 database
.fp5    FileMaker Pro version 5 database