File extensions that begin with R.

 Ext.  Type of file
 R8P   LaserJet portrait font
 R8L   LaserJet landscape font

 RA    RealAudio
 RAM   RealAudio playlist
 RAR   RAR archive
 RAS   Sun bitmap
 RAT   Content ratings
 RAW   Uncompressed digital camera image
 RAW   Uncompressed data (many formats)
 RAW   3D file (open standard)

 RC    Resource script

 RDF   RDF meta-data
 RDFS  RDF schema

 REC   Recorder
 REG   Windows Registry keys
 RES   Programming resource

 RFT   DCA/RFT document

 RGB   SGI bitmap

 RIA   Alpharel Group IV bitmap
 RIB   Renderman graphics
 RIC   Roch FaxNet
 RIX   RIX virtual screen

 RLC   CAD Overlay ESP (Image Systems)
 RLE   Compressed (run length encoded)
 RLF   RichLink compiled format

 RM    RealMedia (RealAudio/RealVideo)
 RMI   MIDI music
 RMM   RealMedia
 RMVB  RealMedia VBR

 RND   AutoShade rendering format
 RNL   GTX Runlength bitmap

 RP    RealPix

 RT    RealText
 RTF   Microsoft text/graphics

 RV    RealVideo