- A ADA programming language source code
- A00 Split Multi-volume ARJ compressed archive (part 2)
- A01 Split Multi-volume ARJ compressed archive (part 3)
- A02 Split Multi-volume ARJ compressed archive (part 4)
- A03 Split Multi-volume ARJ compressed archive (part 5)
- A04 Split Multi-volume ARJ compressed archive (part 6)
- A05 Split Multi-volume ARJ compressed archive (part 7)
- AC Advanced Audio Coding
- AAE Sidecar Image Edit
- AAX Audible enhanced audio
- ABC ABC Programming language
- ABK Ability Write backup
- ABM Album
- ABR Adobe Photoshop Brush
- ABS AB SuDoKu save game
- ACD alan game
- ACH eSignal
- ACL ArchiCrypt Live secured
- ACP Action! document
- ACSM Adobe Content Server Message
- ACT Action! presentation file
- ACTION Mac OS X Automator action
- ACV Adobe photoshop colour curves
- ACX Atari ST Program
- ADC ActiveDocs Clipping Catalog
- ADF A.R.S.E.N.A.L game
- ADT ACT! document template
- AEP Activ E-Book project
- AES:
- Adobe After Effects Stabilizer settings
- AES Crypt encrypted
- AFA Astrotite compressed data
- AFC Apple sound format
- AFF Acorn vector draw
- AFM Abassis data
- AFP Advanced Function Presentation data
- AFR FreeBSD
- AI Adobe Illustrator graphics
- AID ACDSee Classic thumbnail
- AMR Adaptive Multi-Rate compressed audio
- APK Android Package
- ASPX Active Server Page Extended ASP.NET script
- AZ WinDVD related
- AZF Secure format
- AZW Amazon Kindle eBook
- AZW1 Amazon Kindle