Custom widget to upload csv files, in return it posts an article to a KB

There isn't any API for uploading the assets in Li, but there is certainly one for posting the content via API.

restapi/vc/boards/id/[id]/messages/post/?message.subject=this is so cool.

There could be a possible workaround, i.e. you could get the article uploaded in some other application via a widget in your community, and then using the Lithium endpoints, you can make an http call to your uploaded asset and get the content posted via above api in any area of your community.

Hope this will get you started.