It seems like $this->content can also be of type string; however, parameter $array of array_key_exists() does only seem to accept ArrayObject|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
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if (!array_key_exists('filename', /** @scrutinizer ignore-type */ $this->content)) {