Allow upload of arbitrary text files

Summary:
Fixes T8984. Because of how drag-and-drop upload works, the text file with content code is interpreted as a forbidden variable. Disable this check for the drop upload controller.

(The risk here is a general one where the controller redirects and bundles paramters; this controller does not do that, so it's safe to make this change.)

Test Plan: Uploaded a text file containing only the string "code" (no quotes) by using drag-and-drop.

Reviewers: chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T8984

Differential Revision: https://secure.phabricator.com/D13744