This ensures that the content type is also determined when opening JAR channels both synchronously and asynchronously. Current m-c modifies only the async portion, while the mentioned commit below modifies only the sync portion. This commit does it for both.
Partially based on 425677ada7 and bug 1757604.
This reverts commit f62eee7c51.
Verified that a hardening patch landed after this already negates what this patch was a stopgap for, so we no longer need it at all.
This creates a number of stubs and leaves some surrounding code that may be irrelevant (eg. recorded time stamps, status variables).
Stub resolution/removal should be a follow-up to this.