diff --git a/src/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php b/src/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php
index 7b3ae9d160d11..ca69a601cbca2 100644
--- a/src/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php
+++ b/src/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php
@@ -1614,19 +1614,15 @@ public function prepare_item_for_response( $item, $request ) {
 			 * Per-file output format for images, evaluated with the real filename
 			 * and MIME type so plugins filtering image_editor_output_format can
 			 * make per-attachment decisions (e.g. JPEG -> WebP). Resolved the same
-			 * way WP_Image_Editor::set_quality() resolves the output format.
+			 * way WP_Image_Editor::get_output_format() resolves the output format.
 			 */
 			if ( in_array( 'image_output_format', $fields, true ) ) {
 				$filename = get_attached_file( $post->ID );
 
-				/** This filter is documented in wp-includes/media.php */
-				$output_formats = apply_filters(
-					'image_editor_output_format',
-					array( $mime_type => $mime_type ),
+				$output_formats              = wp_get_image_editor_output_format(
 					$filename ? $filename : '',
 					$mime_type
 				);
-
 				$output_mime                 = $output_formats[ $mime_type ] ?? $mime_type;
 				$data['image_output_format'] = ( $output_mime !== $mime_type ) ? $output_mime : null;
 			}
@@ -1643,10 +1639,7 @@ public function prepare_item_for_response( $item, $request ) {
 			if ( in_array( 'image_quality', $fields, true ) ) {
 				$filename = get_attached_file( $post->ID );
 
-				/** This filter is documented in wp-includes/media.php */
-				$output_formats = apply_filters(
-					'image_editor_output_format',
-					array( $mime_type => $mime_type ),
+				$output_formats = wp_get_image_editor_output_format(
 					$filename ? $filename : '',
 					$mime_type
 				);
diff --git a/tests/phpunit/tests/rest-api/rest-attachments-controller.php b/tests/phpunit/tests/rest-api/rest-attachments-controller.php
index 5ae35d5ee370d..46390261862c0 100644
--- a/tests/phpunit/tests/rest-api/rest-attachments-controller.php
+++ b/tests/phpunit/tests/rest-api/rest-attachments-controller.php
@@ -2356,6 +2356,37 @@ static function ( $formats, $filename, $mime_type ) use ( &$captured ) {
 		$this->assertSame( 'image/jpeg', $captured['mime_type'] );
 	}
 
+	/**
+	 * Verifies image_output_format respects filters that use array union (+)
+	 * without overwriting existing keys, ensuring parity with WP_Image_Editor::get_output_format().
+	 *
+	 * @ticket 64677
+	 *
+	 * @covers WP_REST_Attachments_Controller::prepare_item_for_response
+	 */
+	public function test_image_output_format_with_array_union_filter(): void {
+		wp_set_current_user( self::$superadmin_id );
+
+		$attachment_id = self::factory()->attachment->create_upload_object( DIR_TESTDATA . '/images/canola.jpg' );
+
+		add_filter(
+			'image_editor_output_format',
+			static function ( $formats ) {
+				return $formats + array( 'image/jpeg' => 'image/webp' );
+			}
+		);
+
+		$request = new WP_REST_Request( 'GET', '/wp/v2/media/' . $attachment_id );
+		$request->set_param( 'context', 'edit' );
+
+		$response = rest_get_server()->dispatch( $request );
+		$data     = $response->get_data();
+
+		$this->assertSame( 200, $response->get_status() );
+		$this->assertArrayHasKey( 'image_output_format', $data );
+		$this->assertSame( 'image/webp', $data['image_output_format'] );
+	}
+
 	/**
 	 * Verifies image_save_progressive surfaces the filter result for the
 	 * attachment's MIME type.