Add inline attachments to comments

If incoming email is configured and an email is sent, inline
attachments are currently not added to the comment if it has the
`Content-Disposition: inline` instead of
`Content-Disposition: attachment` as e.g. with Apple Mail.

This adds inline attachments (`Content-Disposition: inline`) that have a
filename as attachment to the comment.

Fixes #3496

services/mailer/incoming/incoming.go

@@ -367,6 +367,14 @@ func getContentFromMailReader(env *enmime.Envelope) *MailContent {
 			Content: attachment.Content,
 		})
 	}
+	for _, inline := range env.Inlines {
+		if inline.FileName != "" {
+			attachments = append(attachments, &Attachment{
+				Name:    inline.FileName,
+				Content: inline.Content,
+			})
+		}
+	}
 
 	return &MailContent{
 		Content:     reply.FromText(env.Text),