Pimcore\Model\Object\ClassDefinition\Data\Relations\AbstractRelations::allowAssetRelation PHP Method

    protected function allowAssetRelation($asset)
    {
        $allowedAssetTypes = $this->getAssetTypes();
        $allowedTypes = [];
        $allowed = true;
        if (!$this->getAssetsAllowed()) {
            $allowed = false;
        } elseif ($this->getAssetsAllowed() and is_array($allowedAssetTypes) and count($allowedAssetTypes) > 0) {
            //check for allowed asset types
            foreach ($allowedAssetTypes as $t) {
                if (is_array($t) && array_key_exists("assetTypes", $t)) {
                    $t = $t["assetTypes"];
                }
                if ($t) {
                    if (Admin::isExtJS6()) {
                        if (is_string($t)) {
                            $allowedTypes[] = $t;
                        } elseif (is_array($t) && count($t) > 0) {
                            if (isset($t["assetTypes"])) {
                                $allowedTypes[] = $t["assetTypes"];
                            } else {
                                $allowedTypes[] = $t;
                            }
                        }
                    } else {
                        if (is_string($t)) {
                            $allowedTypes[] = $t;
                        }
                    }
                }
            }
            if (!in_array($asset->getType(), $allowedTypes)) {
                $allowed = false;
            }
        } else {
            //don't check if no allowed asset types set
        }
        Logger::debug("checked object relation to target asset [" . $asset->getId() . "] in field [" . $this->getName() . "], allowed:" . $allowed);
        return $allowed;
    }