What you’re asking for is for the server to push an update to the client, in vanilla Dash this doesn’t exist. There may be some third party libraries that support it, e.g. Live updating — django-plotly-dash documentation but I have never used them.
However on one of my internal apps (and therefore very low latency between client and server) I use dcc.Interval set to run 5 times a second (but it doesn’t even have to be that fast, once a second is probably fine). You can keep track of if the file has updated by checking the mtime and keeping that in dcc.Store, if there isn’t an update you simply return dash.no_update. This really does appear at the front end like the page is updates as soon as the file has updated.