| .B1J | R.K. West Consulting | Uncommon Files | BookJones File | BookJones | R.K. West Consulting |
| .B1N | Unknown Developer | Uncommon Files | Binary Screen Image File | Unknown Software | Unknown Developer |
| .B5B5 | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .B7S | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .BAI | B2W Software | Uncommon Files | BUILD2WIN Add-in | BUILD2WIN | B2W Software |
| .BAK | Various Developers | Uncommon Files | General Backup File | Microsoft SQL Server | Microsoft Corporation |
| .BAO | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .BC | BitComet Development Group | Uncommon Files | BitComet Partially Downloaded File | BitComet | BitComet Development Group |
| .BC! | BitComet Development Group | Uncommon Files | BitComet Incomplete File | BitComet | BitComet Development Group |
| .BDCFG | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .BDMB | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .BDR | Microsoft Corporation | Uncommon Files | Exchange Non-Delivery Report Body File | Microsoft Exchange Server | Microsoft Corporation |
| .BDSM | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .BDU | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .BDZ | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .BER | Unknown Developer | Uncommon Files | German Bericht Report File | Unknown Software | Unknown Developer |
| .BF4 | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .BFAB | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .BFC | Microsoft Corporation | Uncommon Files | Windows Briefcase File | Microsoft Windows Briefcase | Microsoft Corporation |
| .BFCE | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .BFH | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .BGT | FKJ Software | Uncommon Files | Graphics Accounts Budget Data File | Graphic Accounts | FKJ Software |
| .BHB | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .BIE | XnSoft | Uncommon Files | JBIG Image | XnView MP | XnSoft |
| .BIN | Unknown Developer | Uncommon Files | Binary Disk Image File | IsoBuster | Smart Projects |
| .BKD | Kaspersky Lab | Uncommon Files | Book Library Local Dos Exploit Data | Kaspersky Anti-Virus | Kaspersky Lab |
| .BKH | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .BKT | Zortech | Uncommon Files | Zortech Data File | Unknown Software | Unknown Developer |
| .BLF | Microsoft Corporation | Uncommon Files | CLFS Base Log File | Microsoft Common Log File System (CLFS) | Microsoft Corporation |
| .BLI | Thomson Reuters Corporation | Uncommon Files | Thomson Speedtouch Firmware File | Unknown Software | Unknown Developer |
| .BMC | Microsoft Corporation | Uncommon Files | Bitmap Cache File | Microsoft Windows | Microsoft Corporation |
| .BMK | return7, LLC | Uncommon Files | BillMinder Backup File | return7 BillMinder | return7, LLC |
| .BMW | Buzz Tools | Uncommon Files | BuzzPlay Music With Waves | Buzz | Buzz Tools |
| .BM_ | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .BND | IBM | Uncommon Files | DB2 CLI Bind File | IBM DB2 | IBM |
| .BN_ | Unknown Developer | Uncommon Files | Ntgraph Ze32v215 File | Unknown Software | Unknown Developer |
| .BORLAND | Micro Focus International plc | Uncommon Files | Borland C Makefile | Borland C++ | Micro Focus International plc |
| .BOX | Unknown Developer | Uncommon Files | E-mail Mailbox | Mozilla Thunderbird | Mozilla |
| .BPDX | Adobe Systems Incorporated | Uncommon Files | Acrobat Batch PDX File | Adobe Acrobat DC | Adobe Systems Incorporated |
| .BPI | IBM | Uncommon Files | IBM Voice Type Languages New User File | IBM Voice Type | IBM |
| .BPL | Autodesk, Inc. | Uncommon Files | AutoCAD Batch Plot File | Autodesk AutoCAD | Autodesk, Inc. |
| .BPS | Symantec Corporation | Uncommon Files | BPS Programs Virus File | Kaspersky Anti-Virus | Kaspersky Lab |
| .BPU | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .BPX | Truevision Inc. | Uncommon Files | Truevision Targa Bitmap Image | XnView MP | XnSoft |
| .BQY | Oracle Corporation | Uncommon Files | Brio Query File | Internet Explorer | Microsoft Corporation |
| .BRHQUJL | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .BST | Various Developers | Uncommon Files | BibTeX Style Document | TeXworks | Jonathan Kew and Stefan Löffler |
| .BTL | Inmos International plc | Uncommon Files | INMOS Bootable File | INMOS | Inmos International plc |
| .BTS | BitTorrent | Uncommon Files | BitTorrent Sync Unfinished Download File | BitTorrent Sync | BitTorrent |
| .BUB | Apple | Uncommon Files | Photobubble File | Apple QuickTime Player | Apple |
| .BUP | Various Developers | Uncommon Files | Backup File | Zapback | Zaplots Games, Inc. |
| .BUSYBOX | DigiSoft Innovations | Uncommon Files | An Unknown Apple II File | Golden Orchard | DigiSoft Innovations |
| .BUT | ACD Systems | Uncommon Files | Canvas Web Button (CD Audio Track) | ACD Systems Canvas | Microsoft Corporation |
| .BUTTHEREIS | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .BVHJBVHJ | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .BWK | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .BWV | Unknown Developer | Uncommon Files | Business Wave File | Unknown Software | Unknown Developer |
| .BXF | PedFast Technologies | Uncommon Files | Breeder eXchange Format | Man's Best Friend Software | PedFast Technologies |
| .BXM | NetMedia, Inc. | Uncommon Files | BasicX-24 Basic Express Compiler Map | BasicX Programming Environment | NetMedia, Inc. |
| .BXY | Andy Nicholas | Uncommon Files | NuFX Archive File | ShrinkIt | Andy Nicholas |
| .BYL | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .BYZ | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
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