The text Content-Type is intended for sending material which is
principally textual in form. It is the default Content-Type. A
"charset" parameter may be used to indicate the character set of the
body text for some text subtypes, notably including the primary
subtype, "text/plain", which indicates plain (unformatted) text. The
default Content-Type for Internet mail is "text/plain; charset=us-
ascii".
Beyond plain text, there are many formats for representing what might
be known as "extended text" -- text with embedded formatting and
presentation information. An interesting characteristic of many such
representations is that they are to some extent readable even without
the software that interprets them. It is useful, then, to
distinguish them, at the highest level, from such unreadable data as
images, audio, or text represented in an unreadable form. In the
absence of appropriate interpretation software, it is reasonable to
show subtypes of text to the user, while it is not reasonable to do
so with most nontextual data.
Such formatted textual data should be represented using subtypes of
text. Plausible subtypes of text are typically given by the common
name of the representation format, e.g., "text/richtext" [RFC-1341].
