Where is FID xxx used?
To show a usage list for a particular FID (file ID) you can visit the following url on your site:

/file/FID/usage


Just replace FID with the file id you want to check the usage for.

You can see an overview of all file usage at /admin/content/file. You can access this page from the admin toolbar at Content > Files.

Here is an example of what the file usage table looks like on that page:



This shows the name of the entity where the file is in use, the entity type and the usage count.