Strip off MIME types when loading content URLs.

Matching behaviour that has been in ContentLoader.java since
approximately day 0. Without this, we can't load Gmail attachments
with the Chrome HTTP stack because of permission errors -- we have
to load exactly the URL Gmail tells us.

See http://b/issue?id=3125650

Change-Id: I764036efd428299b2fcd611b62dc046a40d80a55