A file extension allows a computer's operating system to decide which program is used to open a file. They can also show what type of file something is, such as image, video, audio or text. For example, .gif, .jpg and .png are all image file types.
File extensions starting with "m" are often music or video formats, such as .mov, .mp3 and .mp4, though some that begin with "m" are for text or images, as well. Many file extensions are made specifically for an individual program or software package, including .rar for WinRAR and ,c4d for CINEMA 4D. These specific ones make it easier for the operating system to quickly access the correct program on the computer.
Other formats, such as text documents like .doc, can be opened by multiple text programs, including Microsoft Word and OpenOffice.org Writer. This allows for more freedom for the user when opening files while still giving the operating system enough information to find the correct program. Most extensions default to opening in a specific program, but it is typically easy to change this setting.
It is also possible to change formats within a program during saving, or by uploading the file to an online file converter that can quickly convert materials to a more usable extension type.