Sent attachments are not shown in other clients

## What Happened

I used Mail 6.1.1 to send an e-mail with an attachment. The attachment shows up as included in Mail, but not other clients (only tested Thunderbird). When inspecting the raw e-mail contents, the attachment is included in the data. Perhaps the syntax was not followed properly.

## Expected Behavior

When I send an e-mail with an attachment, the attachment should not only show up in the sent e-mail in Mail, but in other clients as well.

## Steps to Reproduce

1. Send an e-mail to anyone (yourself) with any attachment.
2. Inspect the received e-mail from Mail, the attachment is shown as present.
3. Inspect the same received e-mail from Thunderbird, the attachment is not shown in the UI.

### Logs

N/A

## Platform Information

Mail 6.1.1