[native] Create real thread when image is sent by pasting

Test Plan: Paste an image in pending sidebar and check if the thread was created

Reviewers: ashoat, atul

Reviewed By: ashoat, atul

Subscribers: KatPo, zrebcu411, Adrian, atul, subnub, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D735