| x3s | X3S Data |
| x61 | Lenovo ThinkPad X61 Bios Update |
| x83 | GAEB DA XML Data File |
| xa | Chiron NRG Exported Dictionary |
| xac | GnuCash Data |
| xac | XMetaL Author Customization File |
| xadd | Microsoft Expression Web Add-in |
| xades | XAdES Signature |
| xadml | XML-Based Application Description Information |
| xaf | IMVU Animation |
| xaf | Autodesk 3D Studio MAX XML Animation |
| xaiml | XAIML Chatterbot Database File |
| xao | SALOME Geometry XAO Data |
| xas | Pro/ENGINEER XAS File |
| xbb | DVIPDFMx Output File |
| xbd | Xilinx Xilinx Board Description Format File |
| xbdr | Darkroom Booth Template |
| xbf | XAML Binary File |
| xbin | RegSupreme Pro Licence Data |
| xbn | Xnick Nick Backup Data |
| xc3 | Cast3D 3D Scene Data |
| xcbr | Sonic Workbench XPath Routing Rules |
| xccolortheme | Xcode Color Theme Data |
| xcd | WarpIN Data File |
| xcn | GenePattern XCN Data |
| xco | Xilinx CORE Generator Log |
| xcp | Casio ClassPad 300 Exported ClassPad |
| xcs | TBX eXtensible Constraint Specification |
| xcsl | CineStyle Color Correction Data |
| xcsr | Oracle UCM Content Folio XML |
| xcst | Oracle UCM Content Folio Template |
| xcworkspace | Xcode Workspace Data |
| xdat | XMVB Data File |
| xdb | Norton Antivirus Update |
| xdd | XFIT XDD Format Data |