Icons can be dragged anywhere on the Dock. There are two types of dragging, smooth dragging and rough dragging, and various types of usage with respect to the dragging style.
Smooth drag is usual way of dragging, and no need extra effort to drag into the dock, e.g. pressing additional key while dragging. However, the action must be performed in a correct way. The right files must be dragged into the right Application (or to the Trash, maybe?).
For example, dragging image file to open in Photoshop
Rough drag is a kind of forcing Application to open files that isn’t suitable. Command-Option must be pressed while dragging, in order to perform this action.
It’s such a refreshing scene when trying to open html file into System Preferences icon on Dock. The title is missing.