MLT across different documents

Hello,

I'm wondering if the following query should be possible -
http://localhost:9200/test_index/article/1119/_mlt?search_types=image&fields=title&size=1

What I'm looking for is the ability to get more documents of type
image based on the article.title field for the specific id. The
documentation seems a little vague on the point of search_types.

The size doesn't seem to work either for a standard _mlt query.

I'm using version 0.16.2

Cheers,
-Ed