Currently only the parameter file name and the directory can be overridden from a taskflow. As such, you would not be able to override a file selected using the cloud hosted option.
Are you able to use flatfile connection? since in case of cloud hosted Agent the local directories wont be accessible,hence the parameter file wont work .However when you publish a taskflow and declare input fields the same can be used through out the execution of all different tasks. Please have a look on the below .
Also you can use REST API to edit the inout parameter values for a mapping task.