Revised June 11, 2018
Some authors have reported that drag-and-drop of files into the Digital Assets Manager (DAM) fails. Instead the files load into the browser as a preview. This occurs when the DAM is accessed through the Websites Admin Console instead of the preferred Digital Assets Manager console.
We do not recommend accessing the DAM through the Websites Admin Console, because it does not provide access to the DAM's full functionality. In this case, there is no built in file upload feature, so instead of uploading your file to the DAM, the browser simply loads the file into the browser -- this is normal functionality.
Since this is normal browser behavior, there is no workaround. We do not plan to change how the DAM functions outside of the Digital Assets Manager console.