.onion isn't actual content right, its just a domain tld that tor uses for its own address spaces. We don't acutally have anything stopping .onion from working far as I know. you will need to have supported file extensions for the directhttp plugin to pick up http links. For example ....onion/file.txt
else you can create linkcrawler rules to support content you want to crawl
more info on rules can be found
https://board.jdownloader.org/showthread.php?t=77280