[BUGFIX] Fix UIDs for files depending on class implementing FileInterface

[dvdmlln]

When using a file collection the fileReferenceUid field contains the actual file UID, while the uidLocal field is null. In this case, there is no FileReference, so fileReferenceUid field should be null.
uidLocal should always contain the file UID.

Maybe rename $fileReference variable to $file to avoid confusion?

[coveralls]

Pull Request Test Coverage Report for Build 13521158399

Details

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 73.13%

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Totals
Change from base Build 13456888799:	0.0%
Covered Lines:	1105
Relevant Lines:	1511

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💛 - Coveralls

[lukaszuznanski]

to test by @twoldanski