List of steps to reproduce (step by step, including full links if applicable):

• Open Livre:Anonyme - n° 3 - Eloge de Blaise Pascal - FR-631136102.pdf (an index file on the French Wikisource, but seems reproducible in other language wikisources)

What happens?:
The thumbnail of the PDF file does not appear. It is replaced by the name of the file. Images of individual pages do not appear either (e.g. here).

The PDF file and its thumbnails appear normally on Commons.

What should have happened instead?:
A thumbnail image of page 1 of the PDF file (available here on Commons) should have appeared on the index page on Wikisource.

Software version (if not a Wikimedia wiki), browser information, screenshots, other information, etc.:
French Wikisource. The same problems appears on the English Wikisource (I tried to create the Index page in preview mode, without actually creating the page).

This was mentioned in the French Wikisource. According to the person who uploaded the PDF file, it stopped working when replacing the PDF file with a new version on Commons.