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.
Also, there is a discrepancy between the objects on both sides of `||`. The left side is a `MIME::Type`, but the right side is a `String`.

This is reconciled on the line below by coercing to `String`, but it seems better to call `#to_s` here, and drop it below.