Prior to this commit, `MimeMessageHelper` would accept `Resource`
instances as inline attachments in multipart MIME messages. If the
provided `Resource` implementation returns `null` for `getFileName()`,
the `addInLine` method would fail with a `NullPointerException` as
Jakarta activation fails to detect the content type for a null filename.
This commit falls back on "application/octet-stream" when the filename
is not known for the given resource instead of failing with an
exception.
Fixes gh-33527
412f5f677b