Data Files :

.csv - Comma Separated Values file
.dat - Data File
.efx - eFax Document
.epub - Open eBook File
.pps - PowerPoint Slid Show
.ppt - PowerPoint Presentation
.pptx - PowerPoint Open XML Presentation
.sdf - Standard Data File.
.xm - XML File

Audio File

.aif  - Audio Interchange File Formate
.iff  - Interchange File Formate
.m3u  - Media Playlist File
.m4a  - MPEG-4 Audio File
.mid  - MIDI File
.mp3  - MP3 Audio File
.mpa - MPEG-2 Audio File
.ra  - Real Audio File
.wav  - WAVE audio File
.wma  - Windows Media Audio File

Font Files :

.fnt  - Window Font File
.fon - Generic Font File
.otf - OpenType Font
.ttf  - TrueType Font

Backup File

.bak - Backup File 
.tmp - Temporary File

Web Files

.css Cascading Style Sheet
.html Hypertext Markup Language File
.js JavaScript File
.jsp Java Server Page
.php Hypertext Preprocessor File
.rss Rich Site Summary
.xhtml Extensible Hypertext Markup

Video Files

.3g2 3GPP2 Multimedia File
.3gp 3GPP Multimedia File
.asf Adavnced Systems Formate File
.asx Microsoft ASF Redirector File
.mp4 MPEG-4 Video File
.mpg MPEG Video File
.vob DVD Video Object file
.avi Audio Video Interleave File
.flv Flash Video file
.mov Apple Quicktime Movie
.rm Real Media File
.srt SubRip Subtitle File
.swf Shockwave Flash Movie
.wmv Windows Media Video file

Compressed Files

.7z 7-Dip Compressed File
.rar Winrar Compressed Archive
 .gz gzip Compressed file
.zip Zipped File
.rpm Red Hat Package Manager File