How can the data in the dash callback function share to the flask view function

How can the data in the dash callback function share to the flask view function

for example:
how to pass the variable filename in dash callback to flask view function named download

@callback(
Output(“id_upsore_url”,“value”),
Input(“id_up_nj”,“value”),
)
def dash_def(f):
filename=f
pass

@app.server.route(’/download/’)
def download(file):
#there how can i use filename from dash
return send_from_directory(‘mydir’, file)