R1:522b16ed123f

Paste - fix N query problem for file URIs

Summary: grab all the files in one big fetch, rather than serially fetching
them. follow up from D1198.

Test Plan: viewed paste and there were no errors!

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, jungejason, btrahan, epriestley

Differential Revision: 1202