bug: write json files to s3 with content type application/json

[jgadling]

Right now when we try to load JSON files from our bucket in a web browser, they're downloaded instead of being displayed in a browser. This makes debugging some portal features pretty difficult for the backend (and maybe even the frontend!) team.

This is happening because cloudfront is returning content-type: binary/octet-stream headers when it should be returning content-type: application/json headers for json files

We can configure CloudFront with a behavior to always return the correct content type for files with a .json extension:
https://stackoverflow.com/questions/37307132/add-content-type-for-specific-file-types-in-amazon-cloudfront