Full forms for commonly used file types

.a - Assembly source file
.avi - Audio Video Interleave
.bak - Backup File
.bat - Batch Processing File
.bin - Binary File
.bmp - Bitmap Image File
.c - C source files
.cab - Cabinet (installation archive format)
.com - Command File
.cpp - C++ source file
.daa - Direct Access Archive
.dat - Data file or Video CD MPEG File
.dll - Dynamic Link Library
.doc - Document (Word)
.docx - (Word) Open XML Format Document
.exe - Executable file
.fla - Flash Movie Authoring File
.flv - Flash Video File
.gif - Graphic Interchange Format
.hlp - Windows Help File
.htm/.html - Hypertext Markup Language
.ico - Icon Image File
.ini - Initialization/Configuration File
.iso - ISO-9660 CD Disc Image
(ISO - International Organization for Standardization)
.java - Java Source File
.jpeg - Joint Photographic Experts Group
.js - JavaScript
.log - Log File
.m3u - MP3 Uniform Resource Locator (MP3 URL)- Mp3 playlist
.m4a - MPEG-4 Audio Layer
.mp3 - MPEG Audio Stream Layer 3
.mp4/.mp4v - MPEG-4 Video File
.mpeg - Moving Picture Experts Group
.msc - Microsoft Management Console Snap-in Control File
.msi - Windows Installer File
.nfo - Infomation (Text) File
.nrg - Nero Disc Image File (made by Nero AG)
.pdf - Portable Document Format
.png - Portable Network Graphics
.pps - PowerPoint Slideshow
.rar - WinRAR Compressed Archive (Roshal ARchive. Founder Roshan)
.rm - Real Media File
.srt - Subrip - Subtitle file format
.swf - Shockwave Flash (Flash Format File)
.sys - System Configuration File
.thm - Thumbnail (Images) or Theme File
.tif/.tiff - Tagged Image File Format
.tmp - Temporary File
.txt - text file
.vob - Video Object (DVD Video File)
.wav/.wave - WaveForm Audio Format
.wma - Windows Media Audio File
.wmv - Windows Media Video File
.xls - Excel Worksheet(Spreadsheet)
.xlsx - Open XML Format Excel Spreadsheet
.xml - Extensible Markup Language
.zip - Compressed Archive File