fix invalid utf-8 when editing an attachment

just let the browser deal with it. garbage in, garbage out

test plan:
 * upload an html file of < 1MB that has invalid UTF-8 in it (such
   as the byte sequence 0xff)
 * attempt to edit it in the browser
 * it should work

Change-Id: I948b9c9f8dcaa68a67703fc9e6bd04cc5c76cac5
Reviewed-on: https://gerrit.instructure.com/28230
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Reviewed-by: Nick Cloward <ncloward@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>