IIRC, when the URL (on the Mastodon side) is the first line, it pushes a text URL as the heading in Lemmy.

Image attachments work too, if you're only using images.

The issue is you can't have a Lemmy URL, a text heading and a custom thumbnail image in a single post.

This creates problems if you have both a Lemmy community and a Mastodon account and you want post only from the Mastodon account (with the post also going to Lemmy).

I spent a lot of time experimenting with this and I couldn't find a good solution. I stick to posting to Lemmy and Mastodon using separate accounts.