| .R8L | LaserJet landscape font |
| .R8P | LaserJet portrait font |
| .RA | RealAudio |
| .RAM | Real Audio |
| .RAS | Raster graphics - Sun |
| .RAW | Raw file format - bitmap (filename extension). |
| .rc | A filename extension found in many UNIX files (.newsrc, etc.) which means "run commands". |
| .REC | Windows Recorder macro file (filename extension). |
| .REG | Registration file (filename extension). |
| .REP | Report file (filename extension). |
| .RFT | RFT-DCA (filename extension). |
| .RGB | An SGI RGB image file |
| .RIA | Alpharel Group IV raster graphics |
| .RIB | Renderman graphics |
| .RIC | Roch FaxNet |
| .RIF | Fractal Painter bitmap (filename extension). |
| .RIFF | Resource Interchange File Format A multimedia file format; the Microsoft equivalent to Amiga IFF format. |
| .RIX | RIX virtual screen |
| .RLA | Wavefront raster image |
| .RLC | CAD Overlay ESP - Image Systems |
| .RLE | Run Length Encoded |
| .RM | Real Media. |
| .RMI | RIFF MIDI file |
| .RND | AutoShade format |
| .RNL | GTX Runlength raster graphics |
| .RPT | Report file (filename extension). |
| .RTF | Microsoft text and graphics |
| .S | Assembly language file - Unix |
| .S3M | MOD music file (filename extension). |
| .SAM | AmiPro |
| .SAV | Saved game (filename extension). |
| .SBP | IBM Storyboard graphics / Superbase text |
| .SC | Source Code - Paradox |
| .SCM | ScreenCam movie |
| .SCP | Dial-up networking script (filename extension). |
| .SCR | 1. Screensaver - Windows. 2. Screen Layout - dBASE. 3. Script for Apple's Disk Copy utility. |
| .SCT | Screen Capture Text - Lotus |
| .SDL | SmartDraw Library (filename extension). |
| .SEA | Self-Extracting Archive (Stuffit or Compact Pro) |
| .SEG | Segment (of a compressed file saved in segments with Stuffit or Compact Pro) |
| .SEP | TIFF bitmap (filename extension). |
| .SET | Setup parameters |
| .SFL | LaserJet landscape font |
| .SFP | LaserJet portrait font |
| .SFS | PCL 5 scalable font |
| .shar | UNIX shell archive (filename extension), which can be decoded by Unshar. |
| .SHTML | HTML file with embedded server-side includes (SSIs) (filename extension). |
| .SI | SoftImage image file |
| .sig | Signature A .sig file can be set up and automatically attached to outgoing email, newsgroup postings, etc. It includes the name and contact information of the sender, and may also have ASCII art, favorite quotes, etc. |
| .SIT | Stuffit compressed file |
| .SLD | AutoCAD slide |
| .SLK | Symbolic Link (SYLK) spreadsheet (filename extension). |
| .SMI | Self-Mounting Image. This file is a Macintosh Disk Image, similar to an .img file, but it does not require the use of Disk Copy. When the file is downloaded, a disk image appears on the desktop. The disk image will behave like a read-only disk. |
| .SND | Sound file |
| .SNM | Netscape mail (filename extension). |
| .SPD | Speedo scalable font |
| .spx | Screen Peace Extension |
| .SQB | SyQuest Backup (filename extension). |
| .SRC | A file used in the creation of .INI files for configuration settings (filename extension). |
| .STY | Style sheet - Ventura |
| .SVX | Amiga 8SVX sound (filename extension). |
| .SWP | Swap - temporary file (filename extension). |
| .SYD | Sysedit backup file (filename extension). |
| .SYL | SYLK spreadsheet format |
| .SYNU | SDSC Synu image |
| .SYS | System file |