Fix!: The ability to get Content-Type value from attachments (Inbound Parse)

[garydevenay]

Fixes #412

Previously we could not infer the content type of an attachment from it's contents (see #412) . I've updated the ParsedEmail Attachments property to be of (new) type ParsedAttachment. This new struct has it's "Content-Type" header populated on email parse and included tests.

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[thinkingserious]

Thank you for the PR!

Could you please update your branch with the latest updates from main? It looks like I don't have permission to do so.