| mcxe | MedicinesComplete Data |
| mcz | Poser Compressed Material Collection |
| md | Chaos Desktop Metadata |
| md | Sega Genesis ROM MD |
| md2p | MessengerDiscovery M2DP Data |
| md2s | MessengerDiscovery Data |
| md5 | MD5 Checksum |
| md5sum | MD5 Checksum |
| md5sums | MD5 Checksum |
| mdat | REW Measurement Data |
| mdb | RockWorks Borehole Database |
| mdbx | Microsoft Access JET Database |
| mdc | Cognos Cube File |
| mdccache | Microsoft Document Connection Cache Data |
| mdcs | MATLAB Distributed Computing Server Saved Session |
| mdd | Notation Composer Midi Device Description |
| mdd | Quanquest SPSS Questionnaire Definition Data |
| mdd | Xilinx Microprocessor Driver Description |
| mdex | Mifos Data Exchange XML Data |
| mdf | Microsoft SQL Server Database |
| mdg | myPM Merge Data |
| mdif | EEsof Model MDIF File |
| mdinfo | iPhone Backup Information Data |
| mdl | Cognos Exported Model Data |
| mdl | MDL Molfile Data |
| mdl | STAR-CD v4 Model Data |
| mdm | BrainVoyager QX MDM Data |
| mdm | HLM Multivariate Data Matrix |
| mdm | OvenWATCH Dataset |
| mdn | Microsoft Access Blank Database Template |
| mdo | Microsoft IIS Metabase Backup |
| mdp | GROMACS Molecular Dynamics Program |
| mdpd | Trajexia Saved Parameters Data |
| mdq | Database Interface Designer Database Query DaTa |
| mdr | ZX Spectrum Emulator Microdrive Cartridge Data |