feat: only allow png file type

[katrinan029]

Description

Updates the accepted file type in the Dropzone to .png only.
https://2u-internal.atlassian.net/browse/ENT-8782

Test plan

1. npm run start:stage
2. go to a customer's setting's tab https://localhost.stage.edx.org:1991/adam-autoapplied-enterprise/admin/settings/appearance
3. upload a jpg image (sample image below) and verify that you see an error alert "Invalid file type, only png images allowed."
   

For all changes

• Ensure adequate tests are in place (or reviewed existing tests cover changes)

Only if submitting a visual change

• Ensure to attach screenshots
• Ensure to have UX team confirm screenshots

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.60%. Comparing base (932b456) to head (9d162ac).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1316   +/-   ##
=======================================
  Coverage   85.59%   85.60%           
=======================================
  Files         575      575           
  Lines       12678    12678           
  Branches     2683     2689    +6     
=======================================
+ Hits        10852    10853    +1     
+ Misses       1765     1764    -1     
  Partials       61       61           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.