Fix theme editor file upload button for NVDA
Fix theme editor file upload button for NVDA

Closes: CORE-1520

* go to the theme editor using NVDA screenreader
* go to one of the file upload inputs
* it should read something useful to you

Change-Id: I941a4c694f2f8a7558b7fce80a2b5fd848ba8347
Reviewed-on: https://gerrit.instructure.com/156471
Tested-by: Jenkins
Reviewed-by: Clay Diffrient <9dff2e5c98626d20d2848250c411e8009465bb87@instructure.com>
QA-Review: Rohan Cheeniyil <c5036ede97e7aae462d224bd80481028506345cc@instructure.com>
Product-Review: Ryan Shaw <ea3cd978650417470535f3a4725b6b5042a6ab59@instructure.com>

djbender/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms