fix attachment copies >= 5GB

Change-Id: Ide74ea459d7ed38acbb4503399e50448f448eea6
Reviewed-on: https://gerrit.instructure.com/39237
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>