Specify the type returned from FileReader readAsDataURL() as PNG and not TIFF?

I'm getting an image that has been pasted into the content editable div and during paste the types in listed in the Clipboard include "image/png" and "image/tiff".

When I get that image using a FileReader and readAsDataURL the base64 image string it returns is a TIFF, "data:image/tiff,".

Is there a way to specify to the FileReader to return a PNG, "data:image/png"?

Related question here.