cleanup, remove trailing whitespace. [ci skip]
Failing test for issue #14435

MailPreviews with inline attachments error out. They try to render an
HTML or plain text part, but it only sees the attachment part
(`image/png` for example) and a `multipart/relative`, returning an
unexpected array when views expect a `Message::Part`.

Failing test for: https://github.com/rails/rails/issues/14435.
Fix on email with inline attachments preview

If email is of type 'multipart/relative', then we render the `text/html`
part contained in it.

Fix for issue: https://github.com/rails/rails/issues/14435
Image now displays.

Change `img src="cid:` image reference for mail clients (RFC 2392) by
inlining image content (`img src="data:`) (RFC 2397), so it can be
previewed in the browser.

Kudos to @xymbol for helping me.