Fetching Data from a URL
One of the most popular types of custom widgets are the "Fetch from URL" widgets. These widgets grab the data you want to show from somewhere on the internet. As long as the data is formatted to match the widget's API, your custom widgets will updated regularly based on the data you have at that URL.
The trick is getting your file to the internet so that Dash can see it. If you've got a web server, by all means, use that! You can serve a static file, or write a web service.
For most people, though, Dropbox is a great solution. Just move your file to your dropbox folder, right click, and select "Share Dropbox Link".
That copies a link to the file that Dash can see, so then you just have to paste that link into the widget's Data URL box.