Allow putting mp3 sounds in the backpack.

src/lib/backpack/sound-payload.js

@@ -18,6 +18,13 @@ const soundPayload = sound => {
         payload.mime = 'audio/x-wav';
         payload.body = assetDataUrl.replace('data:audio/x-wav;base64,', '');
         break;
+    case 'mp3':
+        payload.mime = 'audio/mp3';
+        // TODO scratch-storage should be fixed so that encodeDataURI does not
+        // always prepend the wave format header; Once that is fixed, the following
+        // line will have to change.
+        payload.body = assetDataUrl.replace('data:audio/x-wav;base64,', '');
+        break;
     default:
         alert(`Cannot serialize for format: ${assetDataFormat}`); // eslint-disable-line
     }