fix message attachments in IE 10

test plan:
* using IE 10, should be able to add attachments when
 composing a message in conversations and send the
 message successfully
* should also work in other browsers as before

closes #CNVS-13369

Change-Id: I28459ffbbd9a58751c26d69ce9c6718d3a05c304
Reviewed-on: https://gerrit.instructure.com/42093
Reviewed-by: Clay Diffrient <cdiffrient@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>