Specify default content type for GitHub releases

GitHub requires a Content-Type header for uploading release assets;
specify a default of application/octet-stream so that unrecognized files
can be uploaded.
Oh, I see. Then either use a tertiary operator `? :` or `if then`. Or, find a `MIME::Type` that returns `application/octet-stream` (https://github.com/halostatue/mime-types/blob/master/type-lists/application.yaml#L2481-L2500) which feels a bit convoluted.