Handle nil Content-Type in Service Desk emails

Stan Hurequested to merge
sh-handle-service-desk-nil-content-type into master

!28865 (merged) changed attachment handling in reply-by-email and Service Desk issues and accidentally caused Content-Type to be a required field. If no Content-Type message is provided, Mail::Message returns nil, and the attachment handler would fail. We now handle this nil value gracefully.

Closes #331658 (closed)

Edited by Stan Hu

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