exportToStorage saves as base64 on web

When using exportToStorage() on web the package saves the result as base64 instead of the actual byte representation required. ie. a PDF document will be saved as base64 and cannot be opened properly.

I managed to fix it locally by changing 'data:,base64,' + base64Encode(bytes) in exportToExternalStorage() in file_picker_web.dart to Uri.dataFromBytes(bytes).toString().