| .A50 | Adobe Systems Incorporated | Ficheiros de Texto | Adobe Authorware Library File | Adobe Authorware | Adobe Systems Incorporated |
| .ABW | Abisource | Ficheiros de Texto | AbiWord Document | AbiWord | Open Source |
| .ACP | Alfresco Software, Inc. | Ficheiros de Texto | Alfresco Content Package | Alfresco | Alfresco Software, Inc. |
| .ACT | Bell Labs | Ficheiros de Texto | ADPCM Compressed Audio File | Unknown Software | Unknown Developer |
| .ADA | Open Source | Ficheiros de Texto | Ada Source Code | Notepad++ | Don Ho |
| .ADW | Dr. K. Srinivasan | Ficheiros de Texto | Adhawin Transliteration Scheme | Adhawin | Dr. K. Srinivasan |
| .AFF | Kingsoft | Ficheiros de Texto | Spellcheck Dictionary Description File | Kingsoft Writer | Kingsoft |
| .AFT | Ancestry.com Inc. | Ficheiros de Texto | Family Tree Database | Software MacKiev Family Tree Maker | Ancestry.com Inc. |
| .AGR | Ableton | Ficheiros de Texto | Ableton Groove File | Ableton Live | Ableton |
| .AIM | AIMMS B.V. | Ficheiros de Texto | AIMMS ASCII Model File | AIMMS | AIMMS B.V. |
| .AKS | Srinivas Annam | Ficheiros de Texto | Aksharamala Keymap Source | Aksharamala | Interscape |
| .AKT | Srinivas Annam | Ficheiros de Texto | Aksharamala Keymap Source | Aksharamala | Interscape |
| .ANS | Open Source | Ficheiros de Texto | ANSI Text File | Microsoft Notepad | Microsoft Corporation |
| .APKG | Damien Elmes | Ficheiros de Texto | Exported Anki Flashcard Deck | Anki | Damien Elmes |
| .APO | Abacre Limited | Ficheiros de Texto | Abacre Paperless Office File | Abacre Paperless Office | Abacre Limited |
| .APRJ | Apache HTTP Project Management Committee | Ficheiros de Texto | ApacheConf Project | Apache Web Server | Apache HTTP Project Management Committee |
| .APT | Apache Software Foundation | Ficheiros de Texto | Almost Plain Text File | Apache OpenOffice | Apache Software Foundation |
| .ARK | Avanquest Software | Ficheiros de Texto | PowerDesk Pro Archive | FreeArc | Bulat Ziganshin |
| .ASC | Symantec Corporation | Ficheiros de Texto | PGP (Pretty Good Privacy) ASCII Armored File | Mozilla Thunderbird | Mozilla |
| .ASCII | Sublime HQ Pty Ltd | Ficheiros de Texto | ASCII Text | Microsoft Notepad | Microsoft Corporation |
| .ASE | Adobe Systems Incorporated | Ficheiros de Texto | Adobe Swatch Exchange File | Adobe Illustrator CC | Adobe Systems Incorporated |
| .ATY | G&A M.C. | Ficheiros de Texto | Association Type Placeholder | Topicscape | G&A M.C. |
| .AW | Microsoft Corporation | Ficheiros de Texto | Answer Wizard File | Microsoft Excel | Microsoft Corporation |
| .AWB | Nokia | Ficheiros de Texto | AMR-WB Audio File | Apple QuickTime Player | Apple |
| .AWD | Microsoft Corporation | Ficheiros de Texto | Microsoft Fax Document | PolyView | Polybytes |
| .AWH | ActiveWord Systems, Inc. | Ficheiros de Texto | ActiveWords Browser Help File | ActiveWords | ActiveWord Systems, Inc. |
| .AWP | Ability Plus Software | Ficheiros de Texto | Ability Write Template | Ability Office | Ability Plus Software |
| .AWT | Abisource | Ficheiros de Texto | AbiWord Template | AbiWord | Open Source |
| .AWW | Ability Plus Software | Ficheiros de Texto | Ability Write Document | Ability Office | Ability Plus Software |
| .AXR | Systems AXIS Ltd. | Ficheiros de Texto | AXIS Diplomat Report | AXIS Diplomat | Systems AXIS Ltd. |
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