The "application" Content-Type is to be used for data which do not
fit in any of the other categories, and particularly for data to be
processed by mail-based uses of application programs. This is
information which must be processed by an application before it is
viewable or usable to a user. Expected uses for Content-Type
application include mail-based file transfer, spreadsheets, data for
mail-based scheduling systems, and languages for "active"
(computational) email. (The latter, in particular, can pose security
problems which must be understood by implementors, and are considered
in detail in the discussion of the application/PostScript content-
For example, a meeting scheduler might define a standard
representation for information about proposed meeting dates. An
intelligent user agent would use this information to conduct a dialog
with the user, and might then send further mail based on that dialog.
More generally, there have been several "active" messaging languages
developed in which programs in a suitably specialized language are
sent through the mail and automatically run in the recipient's
Such applications may be defined as subtypes of the "application"
Content-Type. This document defines two subtypes: octet-stream, and
In general, the subtype of application will often be the name of the
application for which the data are intended. This does not mean,
however, that any application program name may be used freely as a
subtype of application. Such usages (other than subtypes beginning
with "x-") must be registered with IANA, as described in Appendix E.