Enable inline attachments for mailgun.

[astjohn]

This PR enables inline attachments for mailgun.

[astjohn]

At the time of writing, mailgun sets the content-id of the attachment to the supplied filename. I have an open ticket with them to see if there is a way to override this. If not, it's not 100% ideal, but it is a working solution.

[astjohn]

Official stance from mailgun is that they don't currently support specifying content-id. Although not ideal, this remains a working solution until the day they decide to support something like that.

[germsvel]

Thanks for opening this PR @astjohn.

Do you have any links to Mailgun documentation that I could reference to help me review this PR?

[astjohn]

It's been a while. Updating our own branch and coming back around to this. Here's the mailgun docs:
https://documentation.mailgun.com/en/latest/api_reference.html#api-reference
https://documentation.mailgun.com/en/latest/api-sending.html#sending

If you'd like to verify what I went through with Mailgun, feel free to email them. As far as I know, their stance hasn't changed on the matter.

[germsvel]

@astjohn, thanks so much for looping back and rebasing the PR to be up to date!

Would you mind adding a test for the MailgunAdapter so the changes are tested?

I think once we have that, this is good to go. 👍