When using a keyboard to navigate, it is much easier and quicker to activate a button than it is to open a menu to select a more appropriate option. For a user with cognitive impairments and/or a new Canvas user navigating with a mouse, I imagine the same could be true.
Currently, if you click on the buttons for adding images, documents, or media, you are met with an Upload interface with no indication that there is a way to access files already stored in Canvas. Without realizing that there is another place they must go to to embed files that are already uploaded, this could lead to a single file being loaded to Canvas several times by mistake. For the person using a keyboard to navigate who realizes they've gone to the wrong place, they have to then use several extra keystrokes to access the right place.
To overcome these issues, I recommend adding a button to the uploading interface that closes the upload interface and opens the folder view tray when pressed. Below is an image suggesting that the button be placed to the right of the tabs. In the image, I labeled the button "Use Uploaded Files" but more appropriate text may belong there instead. In the Tab order, it might make sense to put this button before the tabbed panel so that users are encouraged to use existing files over creating new ones.
Suggested location of button - to the right of tabs