| mdr | ZX Spectrum Emulator Microdrive Cartridge Data |
| mdsx | MEGA Saved Session |
| mdu | D-Flow FM Model Definition Unstructured Data |
| mdus | MessengerDiscovery Internal Data |
| mdw | Microsoft Access Workgroup Data |
| mdx | dBASE IV Multiple Index |
| meb | PRO100 3D Interior Catalog Element |
| mechdb | ANSYS Mechanical Database |
| meg | MEGA Data |
| mega | Megacubo XML Metafile |
| mem | E-Sword v8 Memory Verses Data |
| mem | Virtual Game Station Memory Card |
| mem | Medical Expense Manager Data |
| memo | S Memo Data |
| memx | E-Sword v9 Memory Verses Data |
| menc | Windows Mobile Encrypted File |
| menuskin | ObjectBar Menu Skin |
| mep | Inor MePro Data |
| meps | MEPS Data |
| meq | Corel WordPerfect Macro Editor Print Queue |
| mes | SpectraMagic NX Data |
| mes | TrimeTool MES Output |
| mesa | MESA Model |
| mesh | Rigs of Rods Mesh Data |
| mesh | Microsoft DirectX D3D Mesh Data |
| mess | Salome-Meca Exported Message Data |
| message | WebSTAR Directory Indexer Message |
| meta | Apache META File |
| meta4 | Metalink Data |
| metadata | Acronis True Image Metadata |
| metadix | Apertium Metadix Dictionary Data |
| metalink | Metalink Data |
| metriclog | Apple iOS Diagnostic Data |
| mets | METS Data |
| mev | TIGR MultiExperiment Viewer Format Data |