Filename extensions should always describe a file type. Your installation program should not rename old or backup files by giving them filename extensions like .001, .BAK, or .XX1. If the file type does not change, the program should give the file a new name. For example, it can use long filenames to change the old version of a filename, such as SAMPLE.DLL being changed to Copyof SAMPLE.DLL.
The following table lists filename extensions currently used in Windows. You should not use these filename extensions, unless your file fits the given type description.
| Extension | Type description |
|---|---|
| 386 | Windows virtual device driver |
| 3GR | Screen grabber for Microsoft® MS-DOS® – based applications |
| GR3 | Windows version 3.0 screen grabber |
| ACM | Audio Compression Manager driver |
| ADF | Administration configuration files |
| ANI | Animated mouse cursor |
| AVI | Video clip |
| AWD | Fax viewer document |
| AWP | Fax key viewer |
| AWS | Fax signature viewer |
| BAK | Backed-up file |
| BAT | MS-DOS batch file |
| BFC | Briefcase |
| BIN | Binary data file |
| BMP | Picture (Windows bitmap) |
| CAB | Windows setup file |
| CAL | Windows Calendar file |
| CDA | CD audio track |
| CFG | Configuration file |
| CNT | Help contents |
| COM | MS-DOS –based program |
| CPD | Fax cover page |
| CPE | Fax cover page |
| CPI | International code page |
| CPL | Control Panel application |
| CRD | Windows Cardfile document |
| CSV | Command-separated data file |
| CUR | Cursor (pointer) |
| DAT | System data file |
| DCX | Fax viewer document |
| DLL | Application extension (dynamic-link library) |
| DOC | WordPad document |
| DOS | MS-DOS – based file (also extension for NDIS2 net card and protocol drivers) |
| DRV | Device driver |
| EXE | Application |
| FND | Saved search results |
| FON | Font file |
| FOT | Shortcut to font |
| GR3 | Windows version 3.0 screen grabber |
| GRP | Program group file |
| HLP | Help file |
| HT | HyperTerminal file |
| ICM | Image color matching (ICM) profile |
| ICO | Icon |
| IDF | MIDI instrument definition |
| INF | Setup information |
| INI | Configuration settings |
| KBD | Keyboard layout |
| LGO | Windows logo driver |
| LIB | Static-link library |
| LNK | Shortcut |
| LOG | Log file |
| MCI | MCI command set |
| MDB | File viewer extension |
| MID | MIDI sequence |
| MIF | MIDI instrument file |
| MMF | Microsoft Mail message file |
| MMM | Animation |
| MPD | Mini-port driver |
| MSG | Microsoft Exchange mail document |
| MSN | The Microsoft Network home base |
| MSP | Windows Paintbrush picture |
| NLS | Natural language services driver |
| PAB | Microsoft Exchange personal address book |
| PCX | Picture (PCX format) |
| PDR | Port driver |
| PF | ICM profile |
| PIF | Shortcut to MS-DOS – based application |
| PPD | PostScript® printer description file |
| PRT | Printer formatted file (result of Print to File option) |
| PST | Microsoft Exchange personal information store |
| PWL | Password list |
| QIC | Backup set for Microsoft Backup |
| REC | Windows Recorder file |
| REG | Application registration file |
| RLE | Picture (RLE format) |
| RMI | MIDI sequence |
| RTF | Document (rich text format) |
| SCR | Screen saver |
| SET | File set for Microsoft Backup |
| SHB | Shortcut into a document |
| SHS | Scrap |
| SPD | PostScript printer description file |
| SWP | Virtual memory storage |
| SYS | System file |
| TIF | Picture (TIFF format) |
| TMP | Temporary file |
| TRN | Translation file |
| TSP | Windows telephony service provider |
| TTF | TrueType font |
| TXT | Text document |
| VBX | Visual Basic control file |
| VER | Version description file |
| VXD | Virtual device driver |
| WAV | Sound wave |
| WPC | WordPad file converter |
| WRI | Windows Write document |
| XAB | Microsoft Mail address book |
You should also investigate filename extensions commonly used by popular applications so that you can avoid creating a new extension that might conflict with them, unless you intend to replace or supersede the functionality of those applications.