Arguments to datasource function ignored for MIME Type

[hairyhenderson]

Whoops:

$ gomplate -d dir=/mydir -i {{ (datasource "dir" "config.json").foo.bar }}
template: <arg>:1:21: executing "<arg>" at <"config.json">: can't evaluate field foo in type string

Also, using a ?type= query param in the arg should work too, since it's (generally) just appended to the datasource URL.