add xlsx mime type to supported box view filetypes
add xlsx mime type to supported box view filetypes

fixes gh-642

test plan:
- verify that the following mime type is supported by the box view api:
  application/vnd.openxmlformats-officedocument.spreadsheetml.sheet
  (see https://developers.box.com/view/#filetypes)

Change-Id: Ia0cfc5a86d9d7c2b433645bc0b2efe1cfefd403f
Reviewed-on: https://gerrit.instructure.com/59596
Tested-by: Jenkins
Reviewed-by: Cameron Matheson <1d572acbfa68c7c6e541c7b840d6b622e5c0dc91@instructure.com>
Product-Review: Cameron Matheson <1d572acbfa68c7c6e541c7b840d6b622e5c0dc91@instructure.com>
QA-Review: Amber Taniuchi <ef4bfd1e6d278f6bc7d4a478f02be2cb9581a0a5@instructure.com>

instructure/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,narigone/canvas,sfu/canvas-lms,HotChalk/canvas-lms,HotChalk/canvas-lms,snehakachroo1/LSI_replica,sfu/canvas-lms,djbender/canvas-lms,redconfetti/canvas-lms,sfu/canvas-lms,grahamb/canvas-lms,HotChalk/canvas-lms,venturehive/canvas-lms,grahamb/canvas-lms,instructure/canvas-lms,snehakachroo1/LSI_replica,redconfetti/canvas-lms,matematikk-mooc/canvas-lms,djbender/canvas-lms,matematikk-mooc/canvas-lms,leftplusrightllc/canvas-lms,leftplusrightllc/canvas-lms,narigone/canvas,SwinburneOnline/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,narigone/canvas,snehakachroo1/LSI_replica,narigone/canvas,instructure/canvas-lms,grahamb/canvas-lms,HotChalk/canvas-lms,fronteerio/canvas-lms,redconfetti/canvas-lms,snehakachroo1/LSI_replica,roxolan/canvas-lms,AndranikMarkosyan/lms,venturehive/canvas-lms,grahamb/canvas-lms,fronteerio/canvas-lms,SwinburneOnline/canvas-lms,sfu/canvas-lms,dgynn/canvas-lms,grahamb/canvas-lms,roxolan/canvas-lms,sfu/canvas-lms,matematikk-mooc/canvas-lms,venturehive/canvas-lms,matematikk-mooc/canvas-lms,venturehive/canvas-lms,fronteerio/canvas-lms,leftplusrightllc/canvas-lms,dgynn/canvas-lms,leftplusrightllc/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,AndranikMarkosyan/lms,redconfetti/canvas-lms,roxolan/canvas-lms,SwinburneOnline/canvas-lms,dgynn/canvas-lms,roxolan/canvas-lms,instructure/canvas-lms,AndranikMarkosyan/lms,AndranikMarkosyan/lms,dgynn/canvas-lms,SwinburneOnline/canvas-lms,fronteerio/canvas-lms