Manual:$wgThumbnailEpoch/en

Details
If rendered thumbnail files are older than this timestamp, they will be re-rendered on demand as if the file didn't already exist. Update if there is some need to force thumbs and SVG rasterizations to re-render, such as fixes to rendering bugs.

For a quick copy-paste you can use the following line, which was generated using :

$wgThumbnailEpoch = ;

If you are using a Unix-like system you can get the current date by entering the following on the command-line: