[BUGFIX] Fields of type group file are not properly indexed

If an record with a field of type "group", internal_type "file"
should be indexed in the ReferenceIndex an SQL error exception
gets thrown. So the relation to the file isn't represented in the
refindex. Thus those files are also missing in exports created
with system extension impexp.

This is caused by wrong array keys and incorrectly storing a file
relation as DB relation since merge of FAL.

Resolves: #56353
Resolves: #56352
Releases: 6.2, 6.1, 6.0
Change-Id: I78211efcb6bf3032811ef0b0e20ed7f98f9fa8fb
Reviewed-on: https://review.typo3.org/28796
Reviewed-by: Marc Bastian Heinrichs
Tested-by: Marc Bastian Heinrichs