Quoting original submitter:
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I have been experimenting with Claws Mail for calendaring and found that when
cancelling events as an organiser, Claws (or rather the vCalendar plugin)
sends a mail with an inappropriate Content-Type parameter for the scheduling
method.

According to RFC 6047 (iMIP)...

"The [RFC2045] "Content-Type" header field MUST also include the MIME
parameter "method".  The value MUST be the same (ignoring case) as the value
of the "METHOD" property within the iCalendar object."

http://tools.ietf.org/html/rfc6047#section-2.4

However, in a cancellation object, METHOD will be CANCEL, and thus the
Content-Type must also employ a method parameter with the value "CANCEL".
Instead, Claws' vCalendar plugin seems to use "REQUEST".

The upstream code that is probably responsible for this is here:

http://git.claws-mail.org/?p=claws.git;a=blob;f=src/plugins/vcalendar/vcal_manager.c;h=a21c6a859f888fa3f8950e381dc4f9122e9903f3;hb=HEAD#l1238

Here is what is produced:

Content-Type: text/calendar; method=REQUEST; charset="UTF-8"
Content-Transfer-Encoding: 8bit
Message-ID: <20141218231403.3f080abd.paul.boddie@example.com>

BEGIN:VCALENDAR
VERSION
 :2.0
PRODID
 :-//Claws Mail//NONSGML Claws Mail Calendar//EN
CALSCALE
 :GREGORIAN
METHOD
 :CANCEL

Note the mismatch between the Content-Type method value and the iCalendar
METHOD property.
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Thanks in advance,