attachments in posts by category id

getting no attachments when using this URI /api/get_posts?category__in=15

while when I try with single “_” like this
/api/get_posts?category_in=15

then I m getting all attachments but posts are not related to category id 15.

issue is we need featured Image, which is inside attachment array..

any issue in API call ?

https://www.ads-software.com/plugins/json-api/