This commit separates inline attachments from normal attachments when
listing attachments in Action Mailer previews.  For example, attachments
that were previously listed like

  > Attachments: logo.png file1.pdf file2.pdf

will now be listed like

  > Attachments: file1.pdf file2.pdf (Inline: logo.png)

Co-authored-by: Jonathan Hefner <jonathan@hefner.pro>