It seems like $format can also be of type null; however, parameter $format of Phprest\Util\DataStructu...ssResult::__construct() does only seem to accept string, maybe add an additional type check?
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Ignorable by Annotation
)
If this is a false-positive, you can also ignore this issue in your code via the ignore-type annotation
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return new MimeProcessResult($mime, $vendor, $apiVersion, /** @scrutinizer ignore-type */ $format);