create tempfiles with extensions correctly for account reports

ruby 2.1 adds a timestamp to the tempfile name after the prefix,
so we need to send the extension as a suffix, rather than part
of the prefix

Change-Id: I14df71f629ed42e6f2b6f138788243d9b48b6b40
Reviewed-on: https://gerrit.instructure.com/36761
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Rob Orton <rob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>