This is done by mix phx.digest. It takes all static files and copies them to an additional file with the checksum as part of the filename. It also creates a json manifest file in the root, which has a list of mappings of paths → files with checksum.
This is done to make things like multiple versions on CDN easier – a.k.a. pages already loaded can still access the old script even after a deploy – as well as have proper cache busting if you deploy a new version, so the new html includes a new filename, which prevents the browser from falling back to cached resources no longer current.
