лучше используйте X-
This set of top-level media types is intended to be substantially
complete. It is expected that additions to the larger set of
supported types can generally be accomplished by the creation of new
subtypes of these initial types. In the future, more top-level types
may be defined only by a standards-track extension to this standard.
If another top-level type is to be used for any reason, it must be
given a name starting with «X-» to indicate its non-standard status
and to avoid a potential conflict with a future official name.
