It seems like $files can also be of type false; however, parameter $array1 of array_merge() does only seem to accept array, maybe add an additional type check?
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Ignorable by Annotation
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If this is a false-positive, you can also ignore this issue in your code via the ignore-type annotation
The expression return $files could return the type false which is incompatible with the type-hinted return array. Consider adding an additional type-check to rule them out.