how to open Document's library upload file dialog?

Hello,

I have a task list. User can attach file to the the task. What I want to do is to upload that file to the document library instead of saving it as an attachment. I want to display document's library upload or edit dialog to save the attachment. The dialog should appear when save button of task list item is clicked.  After saving file in a document library, both dialogs (task list and document upload) should close.

How can I achieve this?

Thanks.