| Mozilla Thunderbird Mail Summary Format | |
| N/A | |
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Files with the .MSF extension are message index files. They contain parts of messages, namely only the headline and summary. MSF files are stored in the inboxes of e-mail clients and allow for easy access to the message lists.
| .aawdef | Ad-Aware Definitions Format |
| .abr | Adobe Photoshop Brush Format |
| .ac$ | AutoCAD Undo Info Format |
| .acc | DR DOS - ViewMax Format |
| .acl | Microsoft Office Automatic Correction List |
| .acs | Microsoft Agent Character Structured Storage Format |
| .ad | After Dark Screen Saver Format |
| .add | Microsoft Dynamics AX Developer Documentation Format |