Want to know what a [URL=http://www.fileextensions.net]file extension[/URL] is? File extensions are the short suffixes that appear after the filename. They are used to indicate the file type. In their early use, file extensions were used to indicate the generic type of a file. For example, .txt was used for a text file, and file extension .gfx for a graphics file. However, over time, file extensions have become connected with certain programs or even versions of that program. A .doc file indicates a Microsoft Word document, while a .docx file extension means it was created using Word 2007 or a later version.