R1:1efc66a0dde0

Add file.download to Conduit

Summary:
This is required to make "arc patch" and "arc export" support binary changes.

Test Plan:
Called from web console and "arc".

Reviewed By: aran
Reviewers: tuomaspelkonen, jungejason, aran
CC: aran, epriestley
Differential Revision: 326