Process Incoming File

Titanium Mobile 3.1.2 on Android: I'm allowing other apps to open a PDF file in my app. I process the incoming file like this:

var intent = Ti.Android.currentActivity.getIntent();
if (intent && intent.data) {
    var URL = intent.data;
    var file = Ti.Filesystem.getFile(URL);
    if (file.exists()) {
        alert("Sucess");
    }
}

This works when I open a file from the Downloads folder. However, I have noticed that if I open a file from the Gmail app, intent.data is set to a valid URL, but file.exists() is set to false. Why is this happening?