| File Extension | Description |
| B | Batch list |
| B | Grand Theft Auto saved game |
| B1N | Both mono and color binary screen image |
| B30 | Printer font (JLaser - Cordata) |
| B8 | Raw graphics file (one byte per pixel) plane two |
| BA | Binary Archive |
| BAC | Compiled BASIC |
| BAD | Bad file |
| BAD | File which covers bad blocks on disk volume (not to be moved) |
| BAK | Backup file |
| BAL | Music score |
| BALANCE | Balance accounting sheet file |
| BAP | Blaze Audio Wave Information File |
| BAR | Horizontal bar menu object file |
| BAR | Info Bar for GFX2EXE |
| BAR | Broker Archive. Compressed file containing number of other files for deployment.[1] |
| BAS | BASIC language source |
| BAT | Batch file |
| BB | Database backup |
| BBL | Bibliographic reference file |
| BBM | Brush |
| BBP | Playlist |
| BBS | Bulletin Board System announce or text info file |
| BBS | Hudson-style messagebase |
| BC! | Unfinished download |
| BCF | Card data |
| BCH | Batch process object file |
| BCH | Datafile |
| BCO | Outline font description |
| BCP | Borland C++ makefile |
| BCT | Backup dictionary |
| BCW | Environment settings |
| BDB | Bookends database |
| BDC | Dictionary |
| BDEF | Builder Definition |
| BDF | Adobe Glyph Bitmap Distribution Format |
| BDF | Binary update file |
| BDF | Bitmap Distribution Format font file |
| BDF | Datafile |
| BDM | Videoclip |
| BDR | Border |
| BDT | Dictionary |
| BEZ | Outline font description |
| BF | Brainfuck sourcefile |
| BF2 | Bradford 2 font |
| BFC | Briefcase Win95 |
| BFM | Font metrics |
| BFX | Fax |
| BGA | Bitmap graphics |
| BGI | Borland Graphics Interface device driver |
| BH | Blackhole |
| BI | BASIC include file |
| BIB | Bibliography (ASCII) |
| BIB | Database - not compatible with TeX format |
| BIB | Literature database |
| BIF | Binary Image Format b&w graphics file |
| BIK | Bink video file (RAD Video) |
| BIN | Binary file, usually zero-start (.com analog) |
| BIN | SGI Powerflip |
| BIN | Text mode memory dump |
| BIO | BioniX Wallpaper playlist file |
| BIO | OS/2 BIOS |
| BIT | Bitmap X11 |
| BK | Faxbook |
| BK! | Document backup |
| BKn | Timed backup file for document window n |
| BKP | Backup file |
| BKW | Mirror image of font set |
| BLD | BLoaDable picture |
| BLD | SYSGEN Build procedure file |
| BLEND | Blender3D Project File |
| BLG | Backup Log |
| BLK | Temporary file |
| BLP | Blizzard Game Picture |
| BM | BitMap graphics file |
| BM3 | UIQ3 Phone backup |
| BMK | Help Bookmarks |
| BMP | OS/2 or Win graphics format (BitMap Picture) |
| BMT | Ami Pro Button |
| BNA | Data |
| BNK | Adlib instrument bank file |
| BNR | Graphics Banner |
| BOK | Shamela Library book |
| BOO | Book-format |
| BOO | Compressed file ASCII archive created by BOO (msbooasm.arc) |
| BOO | Boo programming language source file. |
| BOT | Bot files, these go with JKB files |
| BPC | Chart |
| BPOLY | Data |
| BPP | Backup application |
| BPT | Bitmap fills file |
| BR | Script |
| BRD | Eagle Layout File |
| BRD | Tanzzle board file |
| BRD | Cadence Allegro board file |
| BRF | Braille |
| BRK | Fax |
| BRK | The Brake! Mailer REXX script |
| BRL | CAD |
| BS2 | Archive |
| BSA | Archive |
| BSC | Compressed Apple II file archive created by BINSCII |
| BSC | Database |
| BSC | Pwbrmake object file |
| BSP | Half-Life\Half-Life 2 map files |
| BSS | Casio Phone base |
| BST | BiblioTex file (BiblioTex=bibliography file) |
| BSY | Busy flag |
| BTM | Batch-to-memory (quick batch) |
| BTN | Makeover Button file |
| BUFR | Binary Universal Form for the Representation of Meteorological Data |
| BUG | Bugs and Problems |
| BUN | LucasArts Bundle File |
| BUP | Backup file |
| BUT | Button definitions |
| BUY | Datafile format |
| BVn | Overflow file below insert point in Doc n |
| BWB | Spreadsheet application |
| BWR | Beware (buglist) |
| BYU | BYU Movie |
| BZ2 | Archive |
| BZM | Map |
| B_W | Black and white graphics file |
