.acr - American College of Radiology file format. This file format is a standard for ACR-NEMA (National Electrical Manufacturers Association) medical image files.

.adi - AutoCAD Device-Independent Binary Plotter format. Generated by AutoCAD as monochrome bitmaps.

.afm - Adobe Font Metrics format. ASCII-based file format used for storing font metric data as human-readable data.

.ai - Adobe Illustrator format. Native format of the Adobe Illustrator program. A variation of the .eps format.

.aif, .aiff - Audio Interchange File Format. This sound format is predominantly used by Silicon Graphics and Macintosh applications. (filename extension)

.ani - Animated cursor file format. Used with animated cursors in Windows-based machines. (filename extension)

.arc - Archive file format. Most commonly associate with DOS-based machines. This format stores 1+ files in a compressed format in an archive file.

.arj - Archive by Robert Jung. This format stores one or more files into a compressed archive.

.art - Clip Art file format. This extension is used with various types of clip art, including First Publisher and Ashton-Tate.

.asc - ASCII image file format. (filename extension)

.asm - Assembly language file format. (filename extension)

.asp - Active Server Page format. An open, compile-free application environment in which you can combine HTML, scripts and reusable ActiveX server components to create dynamic and powerful Web pages. (filename extension)

.au - Audio U-law (pronounced mu-law) file format. This sound format is most populous on the Internet and boasts small file sizes. (filename extension)

.avi - Audio Video Interleaved file format. Windows multimedia video format that interleaves waveform audio and digital video. (filename extension)

.avr - Audio Visual Research file format. Audio format used to play Macintosh sound files. (filename extension)