[BUGFIX] Show unit "Bytes" for file size of Installer test images

Resolves: #49269
Releases: 6.2
Change-Id: I7f14e04c6bfe538336e1a3dc27c448eeea7a19d0
Reviewed-on: https://review.typo3.org/21583
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring