How to Drag and Drop between full screen apps on a Mac using a standard mouse?
Grab (mouse click) the item you want to drag from say the desktop or email.
Hit Command+Tab on the keyboard to switch through the open apps you have running (holding Command down continuously, and hitting Tab repeatedly) until you get to the app you want to drop the file on. KEEP HOLDING DOWN ON THE MOUSE BUTTON WHILE YOU DO THIS.
When the app you want to drag the item to is the foreground app. You should simply be able to drop (let go of the mouse button) the item you’ve been dragging, and it should be copied or attached to that app.