Here is a list of tuples (for python) for all the video extensions (types[x][0]), the descriptor (types[x][1]), and the commonality (types[x][2]). This was harder to put together than it should have been, and I want to put it here for future reference, because I never want to make it again!

types = (('.3gp2','3GPP Multimedia File','Average'),('.3gpp','3GPP Media File','Average'),('.3mm','3D Movie Maker Movie Project','Average'),('.3p2','3GPP Multimedia File','Average'),('.aaf','Advanced Authoring Format File','Average'),('.aep','After Effects Project','Average'),('.aetx','After Effects XML Project Template','Average'),('.ajp','CCTV Video File','Average'),('.amv','Anime Music Video File','Average'),('.amx','Adobe Motion Exchange File','Average'),('.arf','WebEx Advanced Recording File','Average'),('.avb','Avid Bin File','Average'),('.axm','AXMEDIS Object','Average'),('.bdmv','Blu-ray Disc Movie Information File','Average'),('.bik','BINK Video File','Average'),('.bin','Binary Video File','Average'),('.bix','Kodicom Video File','Average'),('.bmk','PowerDVD MovieMark File','Average'),('.box','Kodicom Video','Average'),('.byu','Brigham Young University Movie','Average'),('.camrec','Camtasia Studio Screen Recording','Average'),('.clpi','Blu-ray Clip Information File','Average'),('.cmmp','Camtasia MenuMaker Project','Average'),('.cmmtpl','Camtasia MenuMaker Template','Average'),('.cmproj','Camtasia Project File','Average'),('.cmrec','Camtasia Recording','Average'),('.cvc','cVideo','Average'),('.d2v','DVD2AVI File','Average'),('.d3v','Datel Video File','Average'),('.dat','VCD Video File','Average'),('.dce','DriveCam Video','Average'),('.dck','Resolume Deck File','Average'),('.dir','Adobe Director Movie','Average'),('.dmb','Digital Multimedia Broadcasting File','Average'),('.dmss','VideoWave SlideShow Project File','Average'),('.dpg','Nintendo DS Movie File','Average'),('.dv','Digital Video File','Average'),('.dv-avi','Microsoft DV-AVI Video File','Average'),('.dvx','DivX Video File','Average'),('.dxr','Protected Macromedia Director Movie','Average'),('.dzt','DirectorZone Title File','Average'),('.evo','HD DVD Video File','Average'),('.eye','Eyemail Video Recording File','Average'),('.f4p','Adobe Flash Protected Media File','Average'),('.fbz','FlashBack Screen Recorder Movie','Average'),('.fcp','Final Cut Project','Average'),('.flc','FLIC Animation','Average'),('.flh','FLIC Animation File','Average'),('.fli','FLIC Animation','Average'),('.flx','FLIC Animation','Average'),('.gfp','GreenForce-Player Protected Media File','Average'),('.gl','GRASP Animation','Average'),('.grasp','GRASP Animation','Average'),('.gts','CaptiveWorks PVR Video File','Average'),('.hkm','Havok Movie File','Average'),('.ifo','DVD-Video Disc Information File','Average'),('.imovieproject','iMovie Project','Average'),('.ivf','Indeo Video Format File','Average'),('.ivr','Internet Video Recording','Average'),('.ivs','Internet Streaming Video','Average'),('.izz','Isadora Media Control Project','Average'),('.izzy','Isadora Project','Average'),('.jts','Cyberlink AVCHD Video File','Average'),('.lsf','Streaming Media Format','Average'),('.m1pg','iFinish Video Clip','Average'),('.m21','MPEG-21 File','Average'),('.m2t','HDV Video File','Average'),('.m2ts','Blu-ray BDAV Video File','Average'),('.m2v','MPEG-2 Video','Average'),('.m4u','MPEG-4 Playlist','Average'),('.mgv','PSP Video File','Average'),('.mj2','Motion JPEG 2000 Video Clip','Average'),('.mjp','MJPEG Video File','Average'),('.mnv','PlayStation Movie File','Average'),('.mp21','MPEG-21 Multimedia File','Average'),('.mpgindex','Adobe MPEG Index File','Average'),('.mpl','AVCHD Playlist File','Average'),('.mpls','Blu-ray  Movie Playlist File','Average'),('.mpv','MPEG Elementary Stream Video  File','Average'),('.mqv','Sony Movie Format File','Average'),('.msdvd','Windows DVD Maker Project File','Average'),('.msh','Visual Communicator Project File','Average'),('.mswmm','Windows Movie Maker Project','Average'),('.mtv','MTV Video Format File','Average'),('.mvb','Multimedia Viewer Book Source File','Average'),('.mvd','Movie Edit Pro Movie File','Average'),('.mve','Infinity Engine Movie File','Average'),('.ncor','Adobe Encore Project File','Average'),('.nsv','Nullsoft Streaming Video File','Average'),('.nuv','NuppelVideo File','Average'),('.nvc','NeroVision  Express Project File','Average'),('.ogv','Ogg Vorbis Video  File','Average'),('.ogx','Ogg Vorbis Multiplexed Media  File','Average'),('.pgi','Video Recording File','Average'),('.piv','Pivot Stickfigure Animation','Average'),('.playlist','CyberLink PowerDVD Playlist','Average'),('.pmf','PSP Movie File','Average'),('.prel','Premiere Elements Project File','Average'),('.pro','ProPresenter Export File','Average'),('.pxv','Pixbend Media File','Average'),('.qtch','QuickTime Cache File','Average'),('.qtl','QuickTime Link File','Average'),('.qtz','Quartz Composer File','Average'),('.rdb','Wavelet Video Images File','Average'),('.rec','Topfield PVR Recording','Average'),('.rmp','RealPlayer Metadata Package File','Average'),('.rms','Secure Real Media File','Average'),('.roq','Id Software Game Video','Average'),('.rp','RealPix Clip','Average'),('.rts','RealPlayer Streaming Media','Average'),('.rum','Bink Video Subtitle File','Average'),('.rv','Real Video File','Average'),('.sbk','SWiSH Project Backup File','Average'),('.seq','NorPix StreamPix Sequence','Average'),('.sfvidcap','Sonic Foundry Video Capture File','Average'),('.smi','SMIL Presentation','Average'),('.smk','Smacker Compressed Movie File','Average'),('.spl','FutureSplash Animation','Average'),('.ssm','Standard Streaming Metafile','Average'),('.svi','Samsung Video File','Average'),('.swt','Flash Generator Template','Average'),('.tda3mt','DivX Author Template File','Average'),('.tivo','TiVo Video File','Average'),('.tod','JVC Everio Video Capture File','Average'),('.tp','Beyond TV Transport Stream File','Average'),('.tp0','Mascom PVR Video File','Average'),('.tpd','Cyberlink TOD Video File','Average'),('.tpr','TMPGEnc Project File','Average'),('.tvs','TeamViewer Video Session File','Average'),('.vc1','VC-1 Video File','Average'),('.vcpf','VideoConvert Project File','Average'),('.vcv','ViewCave Video File','Average'),('.vdo','VDOLive Media File','Average'),('.vdr','VirtualDub Signpost File','Average'),('.vfz','Creative Webcam Video Effects File','Average'),('.vgz','DigitalVDO Compressed Video File','Average'),('.vid','Generic Video File','Average'),('.viewlet','Qarbon Viewlet','Average'),('.viv','VivoActive Video File','Average'),('.vivo','VivoActive Video File','Average'),('.vlab','VisionLab Studio Project File','Average'),('.vp6','TrueMotion VP6 Video File','Average'),('.vp7','TrueMotion VP7 Video File','Average'),('.vpj','VideoPad Video Editor Project File','Average'),('.vsp','VideoStudio Project File','Average'),('.w32','WinCAPs Subtitle File','Average'),('.wcp','WinDVD Creator Project File','Average'),('.webm','WebM Video File','Average'),('.wm','Windows Media File','Average'),('.wmd','Windows Media Download Package','Average'),('.wmmp','Windows Movie Maker Project File','Average'),('.wmx','Windows Media Redirector','Average'),('.wp3','Microsoft Photo Story Project File','Average'),('.wpl','Windows Media Player Playlist','Average'),('.wvx','Windows Media Video Redirector','Average'),('.xfl','Flash Movie Archive','Average'),('.zm1','ZSNES Movie 1 File','Average'),('.zm2','ZSNES Movie 2 File','Average'),('.zm3','ZSNES Movie 3 File','Average'),('.zmv','ZSNES Movie File','Average'),('.aepx','After Effects XML Project','Common'),('.bdm','AVHCD Information File','Common'),('.bsf','Blu-ray AVC Video File','Common'),('.camproj','Camtasia Studio Project','Common'),('.cpi','AVCHD Video Clip Information File','Common'),('.divx','DivX-Encoded Movie File','Common'),('.dmsm','VideoWave Movie Project File','Common'),('.dream','Dream Animated Wallpaper File','Common'),('.dvdmedia','RipIt DVD Package','Common'),('.dvr-ms','Microsoft Digital Video Recording','Common'),('.dzm','DirectorZone Menu Template','Common'),('.dzp','DirectorZone Particle Effect File','Common'),('.f4v','Flash MP4 Video File','Common'),('.hdmov','QuickTime HD Movie File','Common'),('.imovieproj','iMovie Project File','Common'),('.m2p','MPEG-2 Program Stream File','Common'),('.m4v','iTunes Video File','Common'),('.mkv','Matroska Video File','Common'),('.mod','Camcorder Recorded Video File','Common'),('.moi','MOI Video File','Common'),('.mpeg','MPEG Movie','Common'),('.mts','AVCHD Video File','Common'),('.mxf','Material Exchange Format File','Common'),('.ogm','Ogg Media File','Common'),('.pds','PowerDirector Script File','Common'),('.prproj','Premiere Pro Project','Common'),('.psh','Photodex Slide Show','Common'),('.r3d','REDCODE Video File','Common'),('.rcproject','iMovie 08 Project','Common'),('.rmvb','RealMedia Variable Bit Rate File','Common'),('.smil','SMIL Presentation File','Common'),('.srt','SubRip Subtitle File','Common'),('.stx','Pinnacle  Studio Project File','Common'),('.swi','SWiSH Project  File','Common'),('.tix','DivX Video Download Activation  File','Common'),('.trp','HD Video Transport Stream','Common'),('.ts','Video Transport Stream File','Common'),('.veg','Vegas Video Project','Common'),('.vf','Vegas Movie Studio Project File','Common'),('.vro','DVD Video Recording Format','Common'),('.wlmp','Windows Live Movie Maker Project File','Common'),('.wtv','Windows Recorded TV Show File','Common'),('.xvid','Xvid-Encoded  Video File','Common'),('.yuv','YUV Video File','Common'),('787','AVTECH  CCTV Video File','Rare'),('.dsy','Besta Video  File','Rare'),('.gvi','Google Video File','Rare'),('.m15','MPEG  Video','Rare'),('.m4e','MPEG-4 Video File','Rare'),('.m75','MPEG  Video','Rare'),('.mmv','MicroMV Video File','Rare'),('.mpeg4','MPEG-4 File','Rare'),('.mpf','MainActor Project File','Rare'),('.mpg2','MPEG-2 Video File','Rare'),('.mpv2','MPEG-2 Video Stream','Rare'),('.rmd','RealPlayer Media File','Rare'),('.scm','Super Chain Media File','Rare'),('.sec','GuinXell Video File','Rare'),('.vp3','On2 Streaming Video File','Rare'),('2.640000001','Ripped Video Data File','Uncommon'),('.3gpp2','3GPP2 Multimedia File','Uncommon'),('.60d','CCTV Video Clip','Uncommon'),('.aet','After Effects Project Template','Uncommon'),('.avd','Movie Edit Pro Video Information File','Uncommon'),('.avs','Application Visualization System File','Uncommon'),('.bs4','Mikogo Session Video Recording','Uncommon'),('.dav','DVR365 Video File','Uncommon'),('.ddat','DivX Temporary Video File','Uncommon'),('.dif','Digital Interface Format','Uncommon'),('.dlx','Sony VDU Video File','Uncommon'),('.dmsm3d','VideoWave 3D Movie Project File','Uncommon'),('.dnc','Windows Dancer File','Uncommon'),('.dv4','Bosch Security Systems CCTV Video File','Uncommon'),('.fbr','Mercury Screen Recording','Uncommon'),('.gvp','Google Video Pointer','Uncommon'),('.iva','Surveillance Video File','Uncommon'),('.lsx','Streaming Media Shortcut','Uncommon'),('.m1v','MPEG-1 Video File','Uncommon'),('.m2a','MPEG-1 Layer 2 Audio File','Uncommon'),('.meta','RealPlayer Metafile','Uncommon'),('.mjpg','Motion JPEG Video File','Uncommon'),('.modd','Sony Video Analysis File','Uncommon'),('.moff','Sony Video Analysis Index File','Uncommon'),('.moov','Apple QuickTime Movie','Uncommon'),('.movie','QuickTime Movie File','Uncommon'),('.mp2v','MPEG-2 Video File','Uncommon'),('.mp4v','MPEG-4 Video','Uncommon'),('.mpe','MPEG Movie File','Uncommon'),('.mpsub','MPlayer Subtitles File','Uncommon'),('.mvc','Movie Collector Catalog','Uncommon'),('.mvp','Movie Edit Pro Video Project File','Uncommon'),('.mys','Vineyard Captured Video File','Uncommon'),('.osp','OpenShot Project File','Uncommon'),('.par','Dedicated Micros DVR Recording','Uncommon'),('.pssd','PhotoSuite Slide Show File','Uncommon'),('.pva','PVA Video File','Uncommon'),('.pvr','Wintal PVR Video File','Uncommon'),('.qt','Apple QuickTime Movie','Uncommon'),('.qtm','Apple QuickTime Movie File','Uncommon'),('.sbt','SBT Subtitle File','Uncommon'),('.scn','Pinnacle Studio Scene File','Uncommon'),('.sml','SMIL Slideshow Presentation','Uncommon'),('.smv','VideoLink Mail Video File','Uncommon'),('.str','PlayStation Video Stream','Uncommon'),('.vcr','ATI Video Card Recording','Uncommon'),('.vem','Meta Media Video E-Mail File','Uncommon'),('.vfw','Video for Windows','Uncommon'),('.vs4','AVTECH CCTV Video Surveillance File','Uncommon'),('.vse','AVTECH CCTV Video','Uncommon'),('.3g2','3GPP2  Multimedia File','Very Common'),('.3gp','3GPP Multimedia File','Very  Common'),('.asf','Advanced Systems Format File','Very  Common'),('.asx','Microsoft ASF Redirector File','Very  Common'),('.avi','Audio Video Interleave File','Very  Common'),('.flv','Flash Video File','Very Common'),('.mov','Apple  QuickTime Movie','Very Common'),('.mp4','MPEG-4 Video File','Very  Common'),('.mpg','MPEG Video File','Very Common'),('.rm','Real Media  File','Very Common'),('.swf','Flash Movie','Very Common'),('.vob','DVD  Video Object File','Very Common'),('.wmv','Windows Media Video  File','Very Common'))

Here would be an example of how to get a short list, for example looking up all the extensions that are very common:

for type in types:

  if type[2] == 'Very Common':

      print type[0],



.3g2 .3gp .asf .asx .avi .flv .mov .mp4 .mpg .rm .swf .vob .wmv

I simply printed them all in a straight line, however you could format them in whatever way you like, or put them into another data structure for better use! I would also suspect that .ogg and .ogv will increase in popularity with HTML5, and will soon belong to the “Very Common” group!