https://en.wikipedia.org/wiki/ICalendar
RFC 5545 replaced RFC 2445 in September 2009 and now defines the standard
text/calendar - the MIME content type iCalendar data has.
a computer file format which allows Internet users
to send meeting requests and tasks to other Internet users,
via email, or
sharing files with an extension of .ics
.ics = to be used for files containing calendaring and scheduling information,
.ifb = for files with free or busy time information
iCalendar-based code such as
GroupDAV and
CalDAV
are now being used more frequently in both client and server software packages.
RFC 4324 "Calendar Access Protocol"