Here is a list of some file extensions and their abbreviations.
Video File Formats:
AVI= Audio Video Interleave.
RTS= Real Time Streaming.
MP4= MPEG-4 video file.
3GPP=3rd Generation Partnership Project.
WMV=Windows Media Video.
DVX= DivX Video.
Audio File Formats:
MP3= MPEG layer lll.
M4A= MPEG-4 Audio File.
AAC= Advanced Audio Coding.
WMA= Windows Media Audio.
WAV= Waveform Audio.
IMY= iMelody Ringtone.
AMR= Adaptive Multi-Rate Codec.
NRT= Nokia Ringtone.
XMF= Extensible Music File.
Image File Formats:
GIF= Graphic Interchangable Format.
JPEG= Joint Photographic Expert Group.
BMP= Bitmap.
WBMP= Wireless Bitmap Image.
PNG= Portable Network Graphics.
M3G= Mobile 3D Graphics.
SWF= Shock Wave Flash.
Mobile Applications File Formats:
JAR= Java Archive. (Java applications extension)
JAD= Java Application Descriptor.
SIS= Symbian OS Installer File. (Nokia application extension)
APK= Application Package (for android application extension)
NTH= Nokia Theme(series 40).
THM= Themes (SonyEricsson).
MMF= Synthetic Music Mobile Application File.
Documents File Formats:
DOC= Document (Microsoft Corporation).
PDF= Portable Document Format.
HTML= Hyper Text Markup Language.
XML= eXtensible Markup Language
CSS= Cascading Style Sheet
WML= Wireless Markup Language.
CHM= Compiled HTML Help (Microsoft)
TXT= Text document
EXE= Executable file
ELF= Executable and Linkable Format (linux executable)