It seems like $fileField can also be of type Silverback\ApiComponents...otation\UploadableField; however, parameter $property of Symfony\Component\Valida...addPropertyConstraint() does only seem to accept string, maybe add an additional type check?
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Ignorable by Annotation
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If this is a false-positive, you can also ignore this issue in your code via the ignore-type annotation
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$metadata->addPropertyConstraint(/** @scrutinizer ignore-type */ $fileField, new Assert\NotNull(['groups' => sprintf('%s:create', $metadata->getClassName())]));