See how Android stock browser handled downloads

See how the old Android stock browser [handled downloads](https://android.googlesource.com/platform/packages/apps/Browser/+/kitkat-release/src/com/android/browser/DownloadHandler.java) and check what we can re-use to improve the internal download handler.

For example, [URLUtil.guessFileName](http://developer.android.com/reference/android/webkit/URLUtil.html) could be used to determine a proper filename automatically, especially choosing the right extension.