MetadataParser: Handle overriding an external media

Fix #110
4 jobs for override_external_media in 12 minutes and 53 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #289259
amd64 debian
asan-ubsan

00:12:52

passed #289256
amd64 debian
linux

00:10:23

passed #289257
win32
win32

00:12:27

passed #289258
win64
win64

00:11:02