All circle members can add documents to their circle and roles from OneDrive, Google Drive, Dropbox or their computer. In some case, it might be required to remove the permission to attach files from their own computer (for security and access purposes).
To activate the option to prevent users from uploading files from their computer:
Click on your profile picture and go to Administration
Go to the Security settings
Click the toggle to the right.
User won't be able to upload files from their computer.