placeholder background for thumbnails being loaded

src/styles/colors.js

@@ -24,6 +24,8 @@ const Colors = {
   StatsAudio: '#f6a637',
   StatsImage: '#ff4a7d',
   StatsOther: '#26bcf7',
+
+  ThumbnailPlaceholder: '#d5d5d5',
 };
 
 export default Colors;

src/styles/fileList.js

@@ -32,6 +32,7 @@ const fileListStyle = StyleSheet.create({
     color: Colors.White,
   },
   thumbnail: {
+    backgroundColor: Colors.ThumbnailPlaceholder,
     width: thumbnailWidth,
     height: thumbnailHeight,
     marginRight: screenWidthPixels <= 720 ? 10 : 12,