| .S1 | Open Source | Uncommon Files | Free Pascal File | Free Pascal | Open Source |
| .S12 | Unknown Developer | Uncommon Files | WISO Steuer-Sparbuch 2012 Data | WISO steuer:Sparbuch 2018 | Unknown Developer |
| .S1Q | SealedMedia | Uncommon Files | QuickTime Movie | Sealed | SealedMedia |
| .S7P | Symantec Corporation | Uncommon Files | SubSeven Malware | AVG Antivirus | AVG Technologies |
| .SAC | Adobe Systems Incorporated | Uncommon Files | Shared Asset Catalog File | Unknown Software | Unknown Developer |
| .SBC | Microsoft Corporation | Uncommon Files | Office Accounting Company Shortcut File | Microsoft Office Accounting | Microsoft Corporation |
| .SBK | SwishZone | Uncommon Files | SWiSH Project Backup File | SWiSH Max | SwishZone |
| .SBWMM | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .SC | Vladimir Romanyuk | Uncommon Files | Space Engine Script | Space Engine | Vladimir Romanyuk |
| .SC$ | Unknown Developer | Uncommon Files | Modem Bitware Fax Disk1 Files | Unknown Software | Unknown Developer |
| .SC? | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .SCHDOC | Altium | Uncommon Files | Altium Designer File | Altium Designer | Altium |
| .SCHDOCPREV | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .SCPS | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .SCRAP | DigiSoft Innovations | Uncommon Files | An Unknown Apple II File | Golden Orchard | DigiSoft Innovations |
| .SCW | Genie | Uncommon Files | Score Writer Document | GenieSoft Score Writer | Genie |
| .SCZ | OpenLimit SignCubes AG | Uncommon Files | OpenLimit SignCubes Zip Container File | OpenLimit CC Sign | OpenLimit SignCubes AG |
| .SDR | Dell Boomi | Uncommon Files | Dell Support File | Unknown Software | Unknown Developer |
| .SDX | Kivuto | Uncommon Files | Kivuto Secure Download Manager File | Kivuto Secure Download Manager | Kivuto |
| .SEARCH-MS | Microsoft Corporation | Uncommon Files | Windows Saved Search File | Microsoft Windows | Microsoft Corporation |
| .SEC | Symantec Corporation | Uncommon Files | Pretty Good Privacy Secret Key Ring | Pretty Good Privacy (PGP) | Symantec Corporation |
| .SECF | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .SEG | SEG (The Society of Exploration Geophysicists) | Uncommon Files | SEG-2 Data File | Unknown Software | Unknown Developer |
| .SEML | Oracle Corporation | Uncommon Files | Sealed EML Document | Oracle IRM Desktop | Oracle Corporation |
| .SENT | DigiSoft Innovations | Uncommon Files | An Unknown Apple II File | Golden Orchard | DigiSoft Innovations |
| .SEO | Link Engine Management Ltd. | Uncommon Files | PC Link File | PC Link | Link Engine Management Ltd. |
| .SEU | Unknown Developer | Uncommon Files | Serials 2000 Encrypted S2K Update | Unknown Software | Unknown Developer |
| .SF | Institut de Recherche et de Coordination Acoustique/Musique (IRCAM) | Uncommon Files | IRCAM Sound File | Awave Studio | FMJ-Software |
| .SFT | Microsoft Corporation | Uncommon Files | Microsoft App-V Sequencer File | Application Virtualization Sequencer | Microsoft Corporation |
| .SFU | Rockford Corporation | Uncommon Files | Omnifi DMP1 System Firmware Update | Omnifi DMP1 | Rockford Corporation |
| .SFV | Unknown Developer | Uncommon Files | Simple File Verification File | QuickSFV | Totally Useful Software Inc |
| .SGC | Secure Computing (SCC) | Uncommon Files | Secure SnapGear Router Configuration | Unknown Software | Unknown Developer |
| .SGCDB | Stenograph | Uncommon Files | CaseCATalyst File | Case CATalyst | Stenograph |
| .SG_ | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .SHORTCUT | Fanix Software | Uncommon Files | Asutype Shortcut | Asutype | Fanix Software |
| .SHS | Microsoft Corporation | Uncommon Files | Microsoft Scrap File | Microsoft Excel | Microsoft Corporation |
| .SHV | HUSQVARNA Viking | Uncommon Files | Husqvarna Viking Embroidery File | BALARAD Embird Studio | Embird Embroidery Software Suite |
| .SHX | Autodesk, Inc. | Uncommon Files | AutoCAD Compiled Shape File | Autodesk AutoCAD Civil 3D | Autodesk, Inc. |
| .SIGNPRO | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .SIO | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .SIS | Valve | Uncommon Files | Steam Game Backup Information File | Steam | Valve |
| .SISX | Symbian Ltd. | Uncommon Files | Symbian OS Installation File | Nokia PC Suite | Nokia |
| .SK | Apple | Uncommon Files | Safari History Index File | Apple Safari | Apple |
| .SKELETON | DigiSoft Innovations | Uncommon Files | An Unknown Apple II File | Golden Orchard | DigiSoft Innovations |
| .SKP | Trimble Inc. | Uncommon Files | SketchUp Document | SketchUp | Trimble Inc. |
| .SKR | Symantec Corporation | Uncommon Files | PGP Private Keyring | PGP Desktop | Symantec Corporation |
| .SLB | Autodesk, Inc. | Uncommon Files | AutoCAD Slide Library | Autodesk AutoCAD | Autodesk, Inc. |
| .SLDM | Microsoft Corporation | Uncommon Files | Microsoft PowerPoint Macro-enabled Open XML Slide | Microsoft Excel | Microsoft Corporation |
| .SLF | Symantec Corporation | Uncommon Files | Symantec License File | Symantec Ghost Solution Suite | Symantec Corporation |
| .SLS | Sublime HQ Pty Ltd | Uncommon Files | Image Playlist | Notepad++ | Don Ho |
| .SMP | MakeMusic, Inc. | Uncommon Files | SmartMusic Performance File | SmartMusic | MakeMusic, Inc. |
| .SNA | Tom Ehlert Software | Uncommon Files | Drive Snapshot Primary Backup File | Tom Ehlert Drive Snapshot | Tom Ehlert Software |
| .SNF | The Open Group | Uncommon Files | UNIX Font | Unix | AT&T Labs |
| .SNM | NetScape | Uncommon Files | Netscape Mail Index | NetScape Mail | NetScape |
| .SNT | Microsoft Corporation | Uncommon Files | Microsoft Windows Sticky Notes File | Sticky Notes | Microsoft Corporation |
| .SNU | AVG Technologies | Uncommon Files | AVG Anti-Virus Data File | AVG Antivirus | AVG Technologies |
| .SO_ | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .SPD | AutoDesSys, Inc. | Uncommon Files | Form-Z Suspended Render Data | AutoDesSys formZ | AutoDesSys, Inc. |
| .SPK | Atari Corporation | Uncommon Files | Deer Hunter Map File | Deer Hunter | Sunstorm Interactive, Inc. |
| .SPLIT | Incredible Bee Ltd | Uncommon Files | Archiver Split File | 7-Zip | Igor Pavlov |
| .SPWB | StorageCraft | Uncommon Files | ShadowProtect Write Buffer | ShadowProtect | StorageCraft |
| .SR0 | Sony | Uncommon Files | SecuROM Analysis File | SecuRom | Sony |
| .SRG | Ketron | Uncommon Files | Ketron Single Registration File | Ketron Software | Ketron |
| .SRS | Microsoft Corporation | Uncommon Files | Outlook Send/Receive Settings File | Microsoft Outlook | Microsoft Corporation |
| .SS4 | MODRATEC | Uncommon Files | SigScribe4 Design File | SigScribe 4 | MODRATEC |
| .SSC | Microsoft Corporation | Uncommon Files | SourceSafe Status File | Microsoft Visual Studio | Microsoft Corporation |
| .SSCA | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .SSD | Stardock Corporation | Uncommon Files | WindowBlinds Skin File List | WindowBlinds | Stardock Corporation |
| .SSD??= | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .SST | Microsoft Corporation | Uncommon Files | Serialized Certificate Store File | Certificate Manager Tool | Microsoft Corporation |
| .SSTMP | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .SSW | Sony Mobile Communications | Uncommon Files | Sony Ericsson Firmware File | SETool2 | Sony Mobile Communications |
| .SSY | Unknown Developer | Uncommon Files | ScreenShare Compressed Data | ScreenShare | Unknown Developer |
| .STD | Oracle Corporation | Uncommon Files | Apache OpenOffice Drawing Template | Apache OpenOffice | Apache Software Foundation |
| .SU | Unknown Developer | Uncommon Files | Country Code Top-level Domain | Unknown Software | Unknown Developer |
| .SUM | AmsterCHEM | Uncommon Files | Scilab Unit Operation Model File | AmsterCHEM MATLAB CAPE-OPEN Unit Operation | AmsterCHEM |
| .SVK | Commodore International | Uncommon Files | Personal Paint File | Amiga Forever | Cloanto Corporation |
| .SVTREPORTER | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .SVU | PC SOFT | Uncommon Files | Adressmn DEFAULT.DBU File | Adressmn DEFAULT.DBU File | PC SOFT |
| .SWD_RAP | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .SWJ | Dassault Systèmes | Uncommon Files | SolidWorks Journal File | SOLIDWORKS | Dassault Systèmes |
| .SWP | Microsoft Corporation | Uncommon Files | Swap File | Microsoft Windows | Microsoft Corporation |
| .SX2 | Haliplex Communications | Uncommon Files | HPXView SONET /SDH Crossconnect Configuration File | HPXView | Haliplex Communications |
| .SXSCH | SIMetrix Technologies | Uncommon Files | SIMetrix Circuit Schematic | SIMetrix | SIMetrix Technologies |
| .SXT | FMJ-Software | Uncommon Files | Propellerheads Reason 2 NN-XT Sound Data | FMJ-Software Awave Audio | FMJ-Software |
| .SYC | The Wireshark Team | Uncommon Files | Sniffer Capture File | Wireshark | The Wireshark Team |
| .SYMLINK | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .S_S | Unknown Developer | Uncommon Files | Eru/erd File | Unknown Software | Unknown Developer |
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