A computer file format in which all eight bits of the byte are used for data. Executable software (machine language programs), most word processing, database, spreadsheet and multimedia files are binary files. However, text and source program files as well as HTML and XML files are ASCII text files, just plain text, not binary.
Attachments Must Be Encoded
The binary vs. ASCII distinction is made when attaching files via email. The Internet's SMTP mail protocol only supported seven of the eight bits because ASCII text was originally a seven-bit format, and email was only text in the beginning. Although most mail servers today are capable of supporting eight bits, older mail servers may not have been upgraded.
As a result, when binary files are attached to email messages, their 8-bit format is converted into a temporary 7-bit format. Encoding formats such as MIME, UUcoding and BinHex are used, and at the receiving end, they convert the 7-bit code back into 8-bit binary files. The 8-bit to 7-bit conversion makes attached files larger as they traverse the Internet. See
binary,
byte and
ASCII.