Pull Request as a supplementary to #34885

Summary of Changes

• adds a hash to the thumb and the preview image URLs
• The hash is generated once in the app's lifecycle (efficient)

Testing Instructions

This relies on #34885 so if it's not already merged you need to apply that code first

Also, you need to set expire headers in your NginX (or Apache or whatever you're using) so the browser could cache files for some given period (eg 1 year)

Edit a file and check that the media manager displays the edited version!

Also, check the media manager's source code

Actual result BEFORE applying this Pull Request

Expected result AFTER applying this Pull Request

Documentation Changes Required

No. Joomla SHOULD cache (and invalidate) all the static assets.
The media manager team had some plans for a db layer but that was never implemented. Something needs to be done in the direction of storing/retrieving related data, both for cache invalidation but most importantly for the external adapters...
I know cache invalidation is hard but the project should try to fix the fundamentals before going to fancy things like Responsive Images with Art Direction which by the way has a number of architectural flaws...