I can confirm an issue with the FBX exporting options, arguably minor but a bit of a headache in my case. As per my finding, it turns out that the _ExportFileAs option only works if it is specified in last position. As a result, @dale’s example above works fine:
But any version of it where _ExportFileAs is not last, is bound to fail. For example
does not produce an ascii file but indeed a binary one, corresponding to the default option.
Now maybe I am missing a good reason for why that would need to be, but identical tests using the stl file format (which also has an option called _ExportFileAs) do not reproduce the same behaviour.