Revert "thumbnails: keep the orignal file name"

This reverts commit e7006be2.

Passing the filename to an untrusted thumbnailer is a potential
information leak, or a possible method to get that thumbnailer
to crash.

The Flatpak sandboxing codepaths unfortunately cannot remap files, so
that's the reason why the original filename is passed.
4 jobs for wip/hadess/revert-filename-leak in 4 minutes and 45 seconds (queued for 3 seconds)
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Status Name Job ID Coverage
  Test
passed build_stable #1588366

00:04:43

passed flawfinder-sast #1588363

00:00:19

passed secret_detection #1588365

00:00:46

passed semgrep-sast #1588364

00:01:24