| X | Source code file (Lex) |
| X01 | Secondary index file (Paradox) |
| X02 | Secondary index file (Paradox) |
| X03 | Secondary index file (Paradox) |
| X04 | Secondary index file (Paradox) |
| X05 | Secondary index file (Paradox) |
| X06 | Secondary index file (Paradox) |
| X07 | Secondary index file (Paradox) |
| X08 | Secondary index file (Paradox) |
| X09 | Secondary index file (Paradox) |
| X16 | Macromedia Extra (program extension), 16 bit |
| X32 | Macromedia Extra (program extension), 32 bit |
| XAR | Corel Xara drawing |
| XAR | Microsoft Excel auto-save files |
| XBM | A MIME 'X11" bitmap image |
| XCF | GIMP document |
| XCI | XML Configuration Information (Adobe enterprise forms product) |
| XDL | XML Schema file |
| XDP | Label file using Avery DesignerPro Label Design) |
| XEF | XGenics eManager XML form and data for eManager |
| XEM | XGenics eManager Metered units / credit definition for forms/file usage in eManager |
| XEP | XGenics eManager File packaging / unpacking information for eManager components |
| XES | XGenics eManager XML definition for UI skins |
| XET | XGenics eManager XML definition for eManager process |
| XEV | XGenics eManager File download definition for autoupdate and delivery of new procedures |
| XEZ | XGenics eManager Template package for eManager |
| XFD | Acu4GL/ACUCOBOL extended file description (from Acucorp) |
| XFN | Printer font file (Ventura Publisher) |
| XFN | Printer font file (Xerox) |
| XFT | XML Forms Template (from Adobe enterprise forms products such as Adobe Forms Designer) |
| XFX | Fax Document (Various) |
| XHTML | eXtensible hypertext markup language |
| XI | Instrument file (FastTracker II) |
| XI | Instrument sample file (ScreamTracker) |
| XIF | Image file (Xerox) (same as TIF) |
| XLA | Archive (Xlib) |
| XLA | Add-in file (Microsoft Excel) |
| XLB | Datafile (Microsoft Excel) |
| XLB | Toolbar file (Microsoft Excel) |
| XLC | Chart file (Microsoft Excel) |
| XLD | Dialogue file (Microsoft Excel) |
| XLK | Backup file (Microsoft Excel) |
| XLL | Dynamic link library (Microsoft Excel) |
| XLM | Macro file (Microsoft Excel) |
| XLR | Microsoft Works spreadsheet or chart |
| XLS | Worksheet file (Microsoft Excel) |
| XLSX | Microsoft Excel Open XML Document |
| XLT | Translation table (Lotus 1-2-3) |
| XLV | VBA module (Microsoft Excel) |
| XLW | Workbook (Microsoft Excel) |
| XM | Music module (MOD) (Fast Tracker 2) |
| XMI | Compressed midi music (eXtended) |
| XML | eXtensible markup language |
| XMOD | Model file (Monarch Pro – Datawatch) |
| XMPZ | XML Miradi Project Zipfile |
| XNF | Network file (Standard) |
| XNK | Shortcut file (Microsoft Exchange) |
| XON | Datafile (Axon) |
| XPM | Adobe Extensible Metadata Platform file format |
| XPRJ | Project file (Monarch Pro – Datawatch) |
| XQT | Macro file (SuperCalc) |
| XQT | Executable file (Waffle) |
| XR1 | Data file (Epic Megagames Xargon) |
| XRF | Cross-reference file (Generic) |
| XSX | Schema view layout description file (Microsoft XML Designer) |
| XTB | External translation table (LocoScript) |
| XTP | Data file (Xtree) |
| XWD | X Window Dump format (SDSC Image Tool) |
| XWF | Works file (Yamaha XG) (MIDI sequencing) |
| XWK | Keyboard mapping file (Crosstalk) |
| XWP | Text file (Xerox Writer) |
| XWP | Session file (Crosstalk) |
| XWS | Xara Webstyle Graphic |
| XX | Compressed ASCII archive (XXENCODE) |
| XXE | Compressed ASCII archive (XXENCODE) |
| XXX | Singer XL embroidery design file |
| XY | Text file (XYWrite) |
| XY3 | Document file (XYWrite III) |
| XY4 | Document file (XYWrite IV) |
| XYP | Document file (XYWrite III Plus) |
| XYW | Document file (XYWrite for Windows 4.x) |
