Support unmapped asset_search_key value in asset file_properties or metadata

Closes #3527118

src/MediaTypeResolver.php

@@ -60,11 +60,13 @@ final class MediaTypeResolver {
     foreach ($this->getMediaTypes() as $media_type) {
       $source = $media_type->getSource();
       $definition = $source->getPluginDefinition();
-      if (!isset($mapping[$definition['asset_search_key']])) {
-        continue;
+      $property_name = $mapping[$definition['asset_search_key']] ?? $definition['asset_search_key'];
+
+      // Look for property in both file_properties and metadata.
+      $property_value = $asset['file_properties'][$property_name] ?? $asset['metadata']['fields'][$property_name] ?? '';
+      if (is_array($property_value)) {
+        $property_value = reset($property_value);
       }
-      $property_name = $mapping[$definition['asset_search_key']];
-      $property_value = $asset['file_properties'][$property_name] ?? '';
 
       // Compare the data type the actual source plugin can handle vs. the type
       // of the current asset in question.