You cannot use the filepicker element in that way - you need to write some code to store the submitted file in a suitable location and then serve the file back to the end user.
You should look at the Moodle docs about this:
https://docs.moodle.org/dev/File_API
https://docs.moodle.org/dev/Using_the_File_API_in_Moodle_forms
and have a look at a simple example:
https://github.com/AndyNormore/filemanager
You should also consider using a 'filemanager' element, instead of a 'filepicker'. The 'filepicker' is designed for files that you upload, process, then throw away (e.g. a CSV of user data that is used to update existing users, then is deleted). The 'filemanager' is designed for files that are uploaded, stored, then served back to end users.