To debug, put a dialog in onResume() so that your app will pause for input (or some other way of doing this). When it starts, use the debugger to attach to your apps process, e.g. in Studio, Run->Attach debugger to Android process
.
For an icon in the chooser, use this:
<intent-filter android:icon="drawable resource"
...
</intent-filter>
For browsing, nothing you can do since you do not "own" the extension, you just advertise an intent for it.