How to test flow.js uploading with Selenium?

I have an AngularJS SPA where the user can upload files using flow.js, more precisely I'm using its wrapper ng-flow.

Now I'm setting the automated e2e tests up with Selenium, but I can't figure out how to test the forementioned uploading mechanism neither when the user clicks the flow-btn element, nor when he uses the drag and drop.

Googling around I found this page which says that the web driver can't recognize the dialog box opened upon clicking the flow-btn, but, since this very last isn't an <input> element, but just a <span> (the only way to use a flow-btn) , I can't use the solution suggested in the linked page.

Any idea about how to test flow.js file uploading with Selenium?