scratch-storage incorrectly assumes all bitmaps are PNGs

[cwillisf]

Expected Behavior

scratch-storage should be able to load JPG and GIF costumes, as present in the Scratch 2.0 costume library.

Actual Behavior

scratch-storage assumes all bitmap costumes are PNGs, so it tries to load JPG and GIF costumes with the wrong exception. This results in a 404 and a failure to load the costume.

Steps to Reproduce

1. Open your browser's network tab
2. Load project 152639677 in a version of scratch-vm or scratch-gui which uses scratch-storage
3. Look for a 404 on this address: https://cdn.assets.scratch.mit.edu/internalapi/asset/4f234884958b11023d44abc010ad50ec.png/get/