While testing the Document enhancements scheduled for inclusion with the 2017.3 Workfront release, we noticed that there is no longer the ability for our users to paste documents from the clipboard.
According to the Workfront Customer Care team (ticket #1111643), they spoke with the product manager over this area and after having looked at their metrics on how much this option was used they have decided to remove this feature from the new documents are.
Our user community uses this feature on a regular basis. I am hopeful that we can get the vote count up to add this feature back to the tool after the 2017.3 release.