| bth | TimeZero Bathy Recorder Data |
| btif | NationsBank Check Image |
| btk | CLwin Tekton Database |
| btmsp | MALDI BioTyper Main Spectra File |
| bto | Bryce Tree Object |
| btoa | Ascii85 Encoded Data |
| btp | Bend-Tech v3 Plate File |
| btpl | BluffTitler Playlist |
| btpx | Bend-Tech v6 Plate File |
| btr | Btrieve Database |
| bts | Blaise Specification |
| btsearch | BitTorrent Search Engine Data |
| btsk | HyperCard Button Task Data |
| btskey | BitTorrent Sync Data |
| btskin | BitTorrent Skin Data |
| btst | Norton SystemWorks Info Data |
| btsync | BitTorrent Sync Data |
| btt | Microsoft BizTalk Tracking Profile |
| bttx | Bend-Tech v6 Part File |
| btu | Diablo BTU Calculator and Reporting Data |
| btv | AppleTalk Desktop Database |
| btw | Bartender Format Data |
| btx | Bend-Tech v3 Part File |
| btx | DB/TextWorks Database Term and Word Index |
| btx | SmartWare Textfile Backup |
| 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 |
| bui | SOBEK Rainfall Series Data |
| buildsetting | Twixl Publisher Build Settings Data |