set contenttype of gradebook_csv attachment to csv
set contenttype of gradebook_csv attachment to csv

when changing the uploaded_data attribute of an
attachment, the content_type must be set again.
without the content_type safari was downloading csvs
as a txt file.

closes CNVS-20553

Change-Id: I65186f2ea39fb752931913158fff92c2b60a2e25
Reviewed-on: https://gerrit.instructure.com/54465
Tested-by: Jenkins
Reviewed-by: Cody Poll <5859f3fb5e0914506e3e2923810f09e59325ddd4@instructure.com>
QA-Review: Michael Hargiss <59e3fde81b8d9a4f5e0cc016a3c2695591912604@instructure.com>
Product-Review: Dylan Ross <7374864f039b76a5b01d8407f92828e3f173309e@instructure.com>

leftplusrightllc/canvas-lms,sfu/canvas-lms,Rvor/canvas-lms,grahamb/canvas-lms,efrj/canvas-lms,leftplusrightllc/canvas-lms,tgroshon/canvas-lms,shidao-fm/canvas-lms,matematikk-mooc/canvas-lms,sfu/canvas-lms,greyhwndz/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,HotChalk/canvas-lms,narigone/canvas,roxolan/canvas-lms,fronteerio/canvas-lms,AndranikMarkosyan/lms,tgroshon/canvas-lms,wrdsb/canvas-lms,utkarshx/canvas-lms,sdeleon28/canvas-lms,sfu/canvas-lms,utkarshx/canvas-lms,sdeleon28/canvas-lms,jay3126/canvas-lms,fronteerio/canvas-lms,venturehive/canvas-lms,instructure/canvas-lms,kyroskoh/canvas-lms,antoniuslin/canvas-lms,utkarshx/canvas-lms,sfu/canvas-lms,snehakachroo1/LSI_replica,AndranikMarkosyan/lms,kyroskoh/canvas-lms,skmezanul/canvas-lms,whalejasmine/canvas-lms,SwinburneOnline/canvas-lms,grahamb/canvas-lms,jay3126/canvas-lms,HotChalk/canvas-lms,whalejasmine/canvas-lms,dgynn/canvas-lms,snehakachroo1/LSI_replica,djbender/canvas-lms,dgynn/canvas-lms,instructure/canvas-lms,narigone/canvas,eCNAP/CANVAS,Jyaasa/canvas-lms,ShreniiNepal/shrenii_lmp,shidao-fm/canvas-lms,efrj/canvas-lms,fronteerio/canvas-lms,Rvor/canvas-lms,matematikk-mooc/canvas-lms,redconfetti/canvas-lms,efrj/canvas-lms,SwinburneOnline/canvas-lms,eCNAP/CANVAS,Jyaasa/canvas-lms,Jyaasa/canvas-lms,instructure/canvas-lms,redconfetti/canvas-lms,dgynn/canvas-lms,whalejasmine/canvas-lms,venturehive/canvas-lms,SwinburneOnline/canvas-lms,matematikk-mooc/canvas-lms,HotChalk/canvas-lms,fronteerio/canvas-lms,sfu/canvas-lms,redconfetti/canvas-lms,skmezanul/canvas-lms,greyhwndz/canvas-lms,wrdsb/canvas-lms,snehakachroo1/LSI_replica,antoniuslin/canvas-lms,skmezanul/canvas-lms,grahamb/canvas-lms,AndranikMarkosyan/lms,roxolan/canvas-lms,antoniuslin/canvas-lms,matematikk-mooc/canvas-lms,eCNAP/CANVAS,greyhwndz/canvas-lms,utkarshx/canvas-lms,juoni/canvas-lms,kyroskoh/canvas-lms,shidao-fm/canvas-lms,redconfetti/canvas-lms,sfu/canvas-lms,wrdsb/canvas-lms,Rvor/canvas-lms,antoniuslin/canvas-lms,ShreniiNepal/shrenii_lmp,leftplusrightllc/canvas-lms,jay3126/canvas-lms,venturehive/canvas-lms,SwinburneOnline/canvas-lms,eCNAP/CANVAS,juoni/canvas-lms,dgynn/canvas-lms,grahamb/canvas-lms,skmezanul/canvas-lms,AndranikMarkosyan/lms,instructure/canvas-lms,juoni/canvas-lms,tgroshon/canvas-lms,instructure/canvas-lms,ShreniiNepal/shrenii_lmp,sdeleon28/canvas-lms,roxolan/canvas-lms,efrj/canvas-lms,snehakachroo1/LSI_replica,wrdsb/canvas-lms,narigone/canvas,ShreniiNepal/shrenii_lmp,djbender/canvas-lms,instructure/canvas-lms,sdeleon28/canvas-lms,narigone/canvas,juoni/canvas-lms,grahamb/canvas-lms,jay3126/canvas-lms,HotChalk/canvas-lms,Rvor/canvas-lms,greyhwndz/canvas-lms,leftplusrightllc/canvas-lms,sfu/canvas-lms,venturehive/canvas-lms,whalejasmine/canvas-lms,shidao-fm/canvas-lms,tgroshon/canvas-lms,kyroskoh/canvas-lms,Jyaasa/canvas-lms,roxolan/canvas-lms