Set Content-Type header for S3 uploads
Set Content-Type header for S3 uploads

Current document updates do not preserve the content type - it is reset to "application/octet-stream"
We should set the correct content type so that browsers know what to do when users access the documents.

mtekel/digitalmarketplace-admin-frontend,alphagov/digitalmarketplace-admin-frontend,alphagov/digitalmarketplace-admin-frontend,alphagov/digitalmarketplace-admin-frontend,mtekel/digitalmarketplace-admin-frontend,alphagov/digitalmarketplace-admin-frontend,mtekel/digitalmarketplace-admin-frontend,mtekel/digitalmarketplace-admin-frontend