| mbox | Unix Mailbox |
| mbp | Mobipocket Reader Data |
| mbp | Modbus Poll Data File |
| mbs | Opera Mail Mailbox |
| mbsa | Microsoft Baseline Security Analyser Report |
| mbsb | Mamut Enterprise Database Backup |
| mbtemplate | Modelbench Scene Template |
| 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 |
| mc3 | MC3D Document |
| mc4 | MicroCase v4 Data File |
| mc4d | Maxon Cinema 4D Object |
| mc5 | Poser 5 Material Collection |
| mc6 | Poser 6 Material Collection |
| mcacc | MCACC Measurement Data |
| mcapm | PocketMine-MP Minecraft Anvil Data |
| mcat | Meerecat File Guard Encrypted Data |
| mcb | NEPLAN Root Network Data |
| mcdata | MCData XML Data |
| mcdb | Media Cache Database |
| mcf | MPRESS Control Format File |
| mcfi | Autodesk Maya Initial Fluid Cache |
| mcfp | Autodesk Maya Fluid Cache Playback |
| mchelp | MadCap Flare DotNet Help Data |
| mcif | Macromolecular CIF File |
| mck | Macro Keys Encrypted Macro |
| mcl | SmartCAM Macro File |
| mcmac | MasterCook Mac Document |
| mcml | Windows Media Center Markup Language File |
| mcpr | Minecraft Replay Mod Replay |
| mcr | General CADD Macro |