| mapx | MapDotNet UX Map File |
| marshal | Marshal Data Migration Model |
| mask | Adobe SpeedGrade Color Mask |
| mat | Kerkythea Material Data |
| material | Rigs of Rods Texture Reference |
| math | AnalyticMath Data |
| matplot | Matplotlib Plot |
| mauve | Mauve Alignment Data |
| maxc | Autodesk 3D Studio MAX Container |
| maxd | PaperPort Annotation Data |
| maxfr | MaxView Web Pointer Data |
| mb1 | Math Blaster 1 Data |
| mb1 | UltraEdit Menus Data |
| mba | Martus Human Rights Bulletin System Data |
| mbak | Quantrix Modeler Backup Data |
| mbb | CARAIBES Bathymetric Data |
| mbdb | iTunes Backup Manifest Database |
| mbdx | iTunes Backup Manifest Database |
| mbg | Microsoft Mail Mailbag |
| mbg | CARAIBES Georeferenced Depth Data |
| mbk | dBASE IV Multiple Index Backup |
| mbl | Vernier Logger Pro Data |
| mbn | Sony-Ericsson Phone Main Firmware |
| mbox | Unix Mailbox |
| mbp | Modbus Poll Data File |
| mbp | Mobipocket Reader Data |
| mbs | Opera Mail Mailbox |
| mbsa | Microsoft Baseline Security Analyser Report |
| mbsb | Mamut Enterprise Database Backup |
| mbu | Sony CAV-M1000ES Memory Backup |
| mbv | Everio MediaBrowser Edit Data |
| mbw | MBRWizard MBR Backup |
| mbx | Eudora Mailbox File |
| mbx | Microsoft Outlook Express v4 Mailbox |
| mc1 | MedCalc Data |