The "cty" (content type) Header Parameter is used by JWS applications to
declare the media type ([IANA.MediaTypes][]) of the secured content (the
payload). This is intended for use by the application when more than one kind
of object could be present in the JWS Payload; the application can use this
value to disambiguate among the different kinds of objects that might be
present. It will typically not be used by applications when the kind of object
is already known. This parameter is ignored by JWS implementations; any
processing of this parameter is performed by the JWS application. Use of this
Header Parameter is OPTIONAL.

Per [RFC 2045][], all media type values, subtype values, and parameter names
are case insensitive. However, parameter values are case sensitive unless
otherwise specified for the specific parameter.

To keep messages compact in common situations, it is RECOMMENDED that producers
omit an "application/" prefix of a media type value in a "cty" Header Parameter
when no other '/' appears in the media type value. A recipient using the media
type value MUST treat it as if "application/" were prepended to any "cty" value
not containing a '/'. For instance, a "cty" value of "example" SHOULD be used
to represent the "application/example" media type, whereas the media type
"application/example;part="1/2"" cannot be shortened to "example;part="1/2"".

[IANA.MediaTypes]: https://www.iana.org/assignments/media-types/media-types.xhtml
[RFC 2045]: https://tools.ietf.org/html/rfc2045