Multipurpose Internet Mail Extensions was originally be developed for e-mails but became a standard for all kinds of applications (see https://en.wikipedia.org/wiki/MIME). The problem to be solved is to mark and identify in a single data stream individual files (parts) Content-Type. The data stream contains : the marking, identification and the data to be transmitted. Typically in emails there is text and some attachments. Additionally it defines a set of transfer encodings which can be used to represent 8-bit binary data using characters from the 7-bit ASCII character set content-transfer-encoding.

Names and values in MIME headers can be encoded:

Subject: =?utf-8?Q?=C2=A1Hola,_se=C3=B1or!?=

is interpreted as

"Subject: ¡Hola, señor!"

Example:

Content-type: multipart/mixed; boundary="frontier"

MIME-version: 1.0

This is a multi-part message in MIME format.

--frontier

Content-type: text/plain

This is the body of the message.

--frontier

Content-type: text/html; encoding=UTF-8

Content-transfer-encoding: base64

PGh0bWw+CiAgPGhlYWQ+CiAgPC9oZWFkPgogIDxib2R5PgogICAgPHA+VGhpcyBpcyB0aGUg

Ym9keSBvZiB0aGUgbWVzc2FnZS48L3A+CiAgPC9ib2R5Pgo8L2h0bWw+Cg==

--frontier--

Mime links applications to type of data. This is not done once per computer, it is done more per software family:

If no file extension is present the program file tries to find out its content.