The thumbnails are detected by the output filter, and information about them is sent to another object for storage, which is then asked about all the thumbnails it needs to include in the export later.

This catches thumbs on artefact/file/download.php and thumb.php, but doesn't yet seem to handle a couple of cases, like when a user has no profile picture.
(cherry picked from commit 6ee10e357e1ce41e9af4c2cd0c4456b82e9c356e)