File extension

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A file extension is a way of showing the type of a computer file, and a clue to what program it should be opened with. File extensions are usually three letters long and come after the name of the file.

Some examples of common file extensions are:

  • TXT files are plain text files
  • BMP are bitmap picture files
  • JPG are picture files in the JPEG format
  • MP3 are music files in the MP3 format
  • AAC are Advanced Audio Coding music files
  • FLAC are Free Lossless Audio Codec
  • ALAC are Apple Losslss Audio Codec
  • WMA are Windows Media Audio sound files
  • MPEG are motion picture encoded video files
  • MKV are Mastroska Video Codec
  • WMV are Windows Media Video files
  • HTM or *HTML are Hyper Text Markup Language files such as web pages
  • PHP are web server scripts which create web pages
  • ODT are Open Document text files
  • DOC are text documents in Microsoft Word format
  • XLS are Microsoft Excel spreadsheet documents
  • PPT are Microsoft Power Point presentation files
  • EXE are Microsoft Windows executable program files
  • DLL are Dynamic Link Libraries, a type of executable file, in MS Windows
  • ZIP are Compressed files (Lempel Ziff algorithm in an archive)
  • Z are Unix / Linux compressed files
  • bz2 are bzip2 block compressed files (very good compression)
  • jar are Java archive files
  • JSP are Java server pages files
  • java are Java source code files
  • class are Java compiled source code files
  • oc are Java run-time library files
  • tar are Unix / Linux tape archive files
  • sh are Unix / Linux shell script files
  • awk are Unix C like pattern processing language source code files
  • sed are Unix stream editor command files
  • lex are Unix / Linux lexical analyzer C code generator specification files
  • c are C programming language source files
    • o are C programming language compiled (object) files
    • cs are C# programming language files
  • html are Hyper-Text Markup Language files
  • rb are Ruby programming language files
  • pl are Perl or Prolog programming language files


There are many other commonly used file extensions.

You can change the program defaults so that the computer knows which program to open each type of file extension with.