| .REALDOWNLO | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .RECOVERYDA | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .REFER | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .REG | Microsoft Corporation | System Files | Registry File | Unknown Software | Unknown Developer |
| .REGDUMP | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .REGISTER | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .REI | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .REL | Nintendo | Game Files | Relocatable Module File | Stage Switcher | Unknown Developer |
| .RELEASE | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .REMOVE | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .REN | Unknown Developer | Uncommon Files | Renamed File | Unknown Software | Unknown Developer |
| .REP?FB_XD_FRAGMENT= | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .REPORTS | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .REQU | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .REQUEST | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .REQW | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .RESONANCEI | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .RESOURCESC | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .RESPECT | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .RETURNTOOL | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .REUNION | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .REVE | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .REWARD | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .REZCODE | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .RFC1896 | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .RFM | Beatnik | Audio Files | Rich Music Format File | Unknown Software | Unknown Developer |
| .RFX | Unknown Developer | Data Files | TouchStar License | Unknown Software | Unknown Developer |
| .RGA | Unknown Developer | Data Files | Residual Gas Analyzer Data File | Unknown Software | Unknown Developer |
| .RGB | Silicon Graphics, Inc. | Raster Image Files | RGB Bitmap | Unknown Software | Unknown Developer |
| .RGBA | Silicon Graphics, Inc. | Raster Image Files | Silicon Graphics Image File Format | Unknown Software | Unknown Developer |
| .RGH | Unknown Developer | Graphic Files | ZZ RouGH Image | Unknown Software | Unknown Developer |
| .RGR | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .RHT | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .RIA | Unknown Developer | Raster Image Files | Alpharel Group IV Raster Graphics File | Unknown Software | Unknown Developer |
| .RIDEOFVALK | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .RII | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .RKP | Devolutions | Settings Files | RemoteKeys Profile File | Unknown Software | Unknown Developer |
| .RLELZW | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .RLO | Unknown Developer | Data Files | Basic Realizer Careal Library File | Unknown Software | Unknown Developer |
| .RLU | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .ROADS | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .ROUT | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .ROUTINES | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .RPSS | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .RPST | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .RPTDOCUMEN | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .RPTTEMPLAT | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .RPU | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .RQZ | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .RTI | Interscape | Audio Files | Real Tracker Instrument File | Real Tracker | Unknown Developer |
| .RTM | Unknown Developer | Audio Files | Real Tracker Module | Real Tracker | Unknown Developer |
| .RTS | Heaventools Software | Developer Files | Resource Tuner Console Script | Real Tracker | Unknown Developer |
| .RTV | NewTek | Video Files | NewTek Uncompressed Video Data | Unknown Software | Unknown Developer |
| .RU) | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .RV2 | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .RVP | Sharp Electronics | System Files | Sharp Blu-ray Player Firmware File | Unknown Software | Unknown Developer |
| .RWJ | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .S12 | Unknown Developer | Uncommon Files | WISO Steuer-Sparbuch 2012 Data | WISO steuer:Sparbuch 2018 | Unknown Developer |
| .S1H | Oracle Corporation | Web Files | Sealed HTML Document | Unknown Software | Unknown Developer |
| .S1Y | Yamaha Corporation | Audio Files | Yamaha EX-series Synthesizer Internal Voice And Performance Data | Unknown Software | Unknown Developer |
| .S2 | Motorola | Data Files | Motorola S-record Format | Unknown Software | Unknown Developer |
| .S2R | Unknown Developer | Data Files | RegMagik File | RegMagik | Unknown Developer |
| .S4T | i4i, Inc | Data Files | S4/TEXT And Tagless Editor Data | Unknown Software | Unknown Developer |
| .SAC | Adobe Systems Incorporated | Uncommon Files | Shared Asset Catalog File | Unknown Software | Unknown Developer |
| .SAF | DeLorme | GIS Files | Street Atlas USA Map File | Unknown Software | Unknown Developer |
| .SAG | Sagem Communication (SAFRAN Group) | Audio Files | Sagem Ringtone Format | Unknown Software | Unknown Developer |
| .SAIF | Safe Software | Graphic Files | Spatial Archive Interchange Format | Unknown Software | Unknown Developer |
| .SAMPLE | Unknown Developer | Data Files | Saved Sample | Unknown Software | Unknown Developer |
| .SASF | NASA | Developer Files | NASA Spacecraft Activity Sequence | Unknown Software | Unknown Developer |
| .SBE | Safer Networking Ltd. | Settings Files | Spybot Exclude Data | Safeboot | Unknown Developer |
| .SBF | Google | System Files | Android System File | SBF Parser | Unknown Developer |
| .SBWMM | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .SC$ | Unknown Developer | Uncommon Files | Modem Bitware Fax Disk1 Files | Unknown Software | Unknown Developer |
| .SC2 | EA Maxis | Game Files | SimCity 2000 City File | Unknown Software | Unknown Developer |
| .SC? | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .SCAN | Unknown Developer | 3D Image Files | Scan Data | Unknown Software | Unknown Developer |
| .SCANNER | Unknown Developer | Raster Image Files | Scanner Output File | Unknown Software | Unknown Developer |
| .SCHDOCPREV | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .SCPS | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .SCX | RIX SoftWorks, Inc. | Raster Image Files | ColoRIX Bitmap Graphic File | Unknown Software | Unknown Developer |
| .SDR | Dell Boomi | Uncommon Files | Dell Support File | Unknown Software | Unknown Developer |
| .SDS | Oracle Corporation | Data Files | OpenOffice.org Chart | SDS Apps | Unknown Developer |
| .SDU | Edwards Systems Technology, Inc. | Data Files | Edwards Systems Technology File | Unknown Software | Unknown Developer |
| .SEB | Franklin Electronic Publishers, Inc. | eBook Files | Franklin eBookMan E-book | Unknown Software | Unknown Developer |
| .SECF | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .SEG | SEG (The Society of Exploration Geophysicists) | Data Files | SEG-2 Data File | Unknown Software | Unknown Developer |
| .SERIAL | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .SEU | Unknown Developer | Uncommon Files | Serials 2000 Encrypted S2K Update | Unknown Software | Unknown Developer |
| .SFE | Unknown Developer | System Files | Logomatic V2 Firmware | Unknown Software | Unknown Developer |
| .SFO | Interscape | Game Files | PSP Game Parameters File | PBP Unpacker | Unknown Developer |
| .SGB | Relic Entertainment | Game Files | Dawn Of War Map File | Unknown Software | Unknown Developer |
| .SGC | Secure Computing (SCC) | Uncommon Files | Secure SnapGear Router Configuration | Unknown Software | Unknown Developer |
| .SG_ | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .SHAR | Unknown Developer | Compressed Files | Unix Shar Archive | shar | Unknown Developer |
| .SHELL | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .SHOP | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .SIGNPRO | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .SIN | Big Huge Games | Game Files | Rise Of Nations Campaign Media File | UnSin | Unknown Developer |
| .SIO | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .SIX | Digital Equipment Corporation | Raster Image Files | DEC LN03+ Sixel Bitmap Image File | Unknown Software | Unknown Developer |
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