i don't think nohtml_start_urls is a good name. The thing to fix is the text, and remove the non-HTML part (as anyway the field accepts also html ones) In fact start_urls could be considered as a backward compatibility, but in new usage can be merged with feed_start_urls, which should in that case change its name. May be start_requests is a better naming (as it is not only an url, but a link extractor).

About using the Content-Type, it is not useful, because we still need to define which link extractor to apply.