Import blog post teaser images as Blog post image thumbnail

Confluence lets users select an image as the blog post teaser.

This ends up being an image with ac:class="teaser" in the body content:

<ac:image ac:class="teaser" ac:width="700"><ri:attachment ri:filename="MyTeaser.png" /></ac:image>

We need to catch this and put the filename as the image parameter of the blog post object.