| xdfl | eXtensible Data Flow Language Document |
| xdfz | Compressed Extensible Data Format File |
| xdoclet | JBoss XDoclet Reference Data |
| xdt | Xscript Data File |
| xfdl | XPLATFORM Screen Form Definition |
| xfl | Adobe Flash Exchange File |
| xgdw | GDevelop Extension |
| xgf | Xgridfit Program |
| xgr | GraphEdit Filter Graph Markup Language |
| xib | Apple Interface Builder Development-Time Resources |
| xil | Xilize Markup Source |
| xilinc | Xilize Include |
| xilize | Xilize Configuration Data |
| xin | OmniMark Include File |
| xje | XJEase Source Code |
| xl | XBLite Source Code |
| xmla | XML for Analysis File |
| xmljet | Eclipse JET XML Template |
| xnb | XNA Game Studio Binary Package |
| xnd | exNode Download Data |
| xob | WeintekHMI EasyBuilder 8000 XOB File |
| xoml | Microsoft Visual Studio Windows Workflow |
| xpb | XPB! Basic Source Code |
| xpk | Xscript Package |
| xpp | X++ Source Code |
| xpz | GeneXus Object |
| xq | XQuery File |
| xql | XML Query Language |
| xqm | Oxygen XML Editor XQuery Module |
| xquery | XQuery File |
| xqy | XQuery File |
| xrc | wxWidgets XML Based Resource |
| xrd | XRK Data File |
| xrg | Xscript Registry Data |
| xrm | XrmToolkit XRM Data |