fix writeBinaryFile in the js api to call the correct function

on the rust side. Before, files were sent in base64 encoding, but

the save-as-text `WriteFile` fs command was reused, without any

indication that the contents were base64 encoded. So the WriteFile

command had no way of checking and decoding when necessary.

WriteBinaryFile on the rust side expects a base64 encoded string, then

decodes it.