Applied to the file name for the thumbnail when uploading an image.
This database has information for all major versions from WP 1.2.1 through 5.1.
|2.0||wp-admin/admin-functions.php||746||View in context||filter|
|2.0.11||wp-admin/admin-functions.php||785||View in context||filter|
|2.1||wp-admin/admin-functions.php||2188||View in context||filter|
|2.2||wp-admin/admin-functions.php||2277||View in context||filter|
|2.3||wp-admin/includes/image.php||72||View in context||filter|
This hook does not occur in the most recent version of WordPress (5.1). Do not use it. It is deprecated. You should look at the list of "related hooks" below to see if you can figure out what replaced it.
We find related hooks using word stems. thumbnail_filename has 2 significant word stem(s):
filename. Note that some of the hooks below may not appear in all versions of WP.