| bto | Bryce Tree Object |
| btp | Bend-Tech v3 Plate File |
| btpx | Bend-Tech v6 Plate File |
| btr | Btrieve Database |
| bts | Blaise Specification |
| btsearch | BitTorrent Search Engine Data |
| btskey | BitTorrent Sync Data |
| btskin | BitTorrent Skin Data |
| btt | Microsoft BizTalk Tracking Profile |
| bttx | Bend-Tech v6 Part File |
| btv | AppleTalk Desktop Database |
| btw | Bartender Format Data |
| btx | Bend-Tech v3 Part File |
| btx | DB/TextWorks Database Term and Word Index |
| btz | Helix Toolkit Compressed BT Data |
| bu | Pegasus Mail BU File |
| bub | PhotoBubble Image Data |
| buc | Adobe Photoshop Elements Backup Catalog |
| bud | Windows Binary Printer Description File |
| buf | Emacs Buffer Data |
| bufr | Binary Universal Form for the Representation |
| bugsense | BugSense Data |
| buk | DVD IFO Backup File |
| bul | I4 Bullseye Data |
| bumptheme | BumpTop Theme Data |
| bunv | eZ-TOMAS Binary Universal File |
| buv | BrightAuthor User Variables Data |
| bv | Open Board Viewer Database |
| bv3 | Corel WordPerfect Overflow Data |
| bva | Biovision Motion Data |
| bval | DTI BVAL Data |
| bvec | DTI BVEC Data |
| bvf | VentaFax Phonebook |
| bvh | Biovision Hierarchy Data |
| bvl | Micrografx Picture Publisher 8 Texture |