Media, REST API: Allow media edit endpoint to find remote files.

Props jonsurrell.
Fixes #50595.

git-svn-id: https://develop.svn.wordpress.org/trunk@48386 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php

@@ -460,7 +460,15 @@ public function edit_media_item( $request ) {
 			);
 		}
 
-		$image_editor = wp_get_image_editor( $image_file );
+		// If the file doesn't exist, attempt a URL fopen on the src link.
+		// This can occur with certain file replication plugins.
+		// Keep the original file path to get a modified name later.
+		$image_file_to_edit = $image_file;
+		if ( ! file_exists( $image_file_to_edit ) ) {
+			$image_file_to_edit = _load_image_to_edit_path( $attachment_id );
+		}
+
+		$image_editor = wp_get_image_editor( $image_file_to_edit );
 
 		if ( is_wp_error( $image_editor ) ) {
 			return new WP_Error(