| x | Microsoft DirectX D3D Object |
| x | XBLite Source Code |
| xadl | XPLATFORM Application Program Definition |
| xal | Exient XGS Asset List Data |
| xal | Open XAL Configuration |
| xamlx | Visual Studio Workflow Service |
| xap | XACT Project File |
| xat | XAT Template |
| xbl | XBLite Source Code |
| xbml | SAP Business One Studio Workflow Process Data |
| xbw | SUP Hybrid Web Container Workflow |
| xcappdata | Xcode App Data |
| xcarchive | Xcode Archive |
| xcb | Visual C++ Command Bar Data |
| xcconfig | Xcode Configuration Settings |
| xccrashpoint | Xcode Crash Point Data |
| xcdatamodel | Xcode Core Data Model |
| xcdatamodeld | Xcode Core Data Model |
| xcent | Xcode Entitlements Data |
| xcl | IAR System Linker Command File |
| xclangspec | Xcode Language Specification Data |
| xcmd | HyperCard External Commands |
| xcodeplugin | Xcode Plug-in |
| xcof | Mac OS Object Library |
| xcp | XLNT Script |
| xcplugin | Xcode Plug-in |
| xcplugindata | Xcode Plug-in Data |
| xcscheme | Xcode Scheme Document |
| xcsnapshots | Xcode Snapshots Data |
| xcss | RichFaces XCSS File |
