MediaGallery: fix filename text color

app/styles/components/media-gallery.scss

@@ -164,7 +164,7 @@
 .media-gallery__file-extension__label {
   display: block;
   position: absolute;
-  color: var(--primary-text-color);
+  color: #fff;
   background: rgba($base-overlay-background, 0.5);
   bottom: 6px;
   left: 6px;