| .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 |
| .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 |
| .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 |
| .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 |
| .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 |
| .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 |
| .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 |
| .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 |
| .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 |
| .SLAP | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .SMART2 | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .SMH | Oracle Corporation | Text Files | Sealed MHT Document | Unknown Software | Unknown Developer |
| .SML2 | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .SMN | Unknown Developer | Data Files | ParaGraph WorkScript Data | Unknown Software | Unknown Developer |
| .SNC | Swiftpage, Inc. | Settings Files | ACT! E-mail/folder Synchronization File | Unknown Software | Unknown Developer |
| .SND | Open Source | Audio Files | Sound File | Unknown Software | Unknown Developer |
| .SNF | The Open Group | Font Files | UNIX Font | Unknown Software | Unknown Developer |
| .SNIFFER | Unknown Developer | Data Files | Sniffer Trace File | Unknown Software | Unknown Developer |
| .SOFTWARE | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .SOUP | Unknown Developer | Data Files | Message / E-mail File | Unknown Software | Unknown Developer |
| .SO_ | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .SPAM | Unknown Developer | Web Files | Spam E-mail Message | Unknown Software | Unknown Developer |
| .SPARSE | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .SPIFF | Joint Photographic Experts Group | Raster Image Files | Still Picture Interchange File Format | Unknown Software | Unknown Developer |
| .SPRITES | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .SRH | BMC | Data Files | BMC Remedy AR System Server Saved Search | Unknown Software | Unknown Developer |
| .SRO | Unknown Developer | Data Files | MADCOW Multi-Site ATIS Data Collection Working Group Data | Unknown Software | Unknown Developer |
| .SS2 | Unknown Developer | Data Files | Terrasoft SAS Program File | Unknown Software | Unknown Developer |
| .SS3 | Unknown Developer | Audio Files | Singer Song Writer Audio File | Unknown Software | Unknown Developer |
| .SSCA | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .SSD??= | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .SSTMP | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .STACK | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .STAGE1 | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .STAR | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .STARTUP | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .STDF | Teradyne | Data Files | Standard Test Data Format | Unknown Software | Unknown Developer |
| .STEGANO | Unknown Developer | Data Files | Steganographic Image File | Unknown Software | Unknown Developer |
| .STORAGE | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
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