Fix media_type to reflect extension
Fix media_type to reflect extension

I wrote `application/json` when making the test without looking at the
extension. This `text/plain` is more sensible for txt.

ahmadassaf/open-data-certificate,Motejl/open-data-certificate,theodi/open-data-certificate,theodi/open-data-certificate,theodi/open-data-certificate,zoul/open-data-certificate,theodi/open-data-certificate,Motejl/open-data-certificate,zoul/open-data-certificate,ahmadassaf/open-data-certificate,ahmadassaf/open-data-certificate,zoul/open-data-certificate,ahmadassaf/open-data-certificate,Motejl/open-data-certificate,Motejl/open-data-certificate,zoul/open-data-certificate