Allow to Drag & Drop files from downloads popover
You download an image from the web and directly want to use this image in another program.
Google Chrome does this very well, which let's me drag the downloaded file and even change window while dragging. Firefox allows me to drag the image, but disallows changing the active window with alt-tab while dragging, which makes it not very useful in most situation.
From a UX point of view, D&D is well established and very intuitive. Especially when working with files, users know the concept and mostly know (or expect) D&D gestures to be available when interacting with an app. This feature would make Epiphany match that expectation.
When the downloads popover is open the use can click and drag the file from the popover into another window (e.g. GIMP or Inkscape). While dragging, the user can also change the active window with the alt+tab keyboard shortcut to fo find her drop target window.
Ideally, dragging the file across workspaces would also be possible with the cursor only (the workspace switch keyboard-shortcuts should work also)
GNOME 3.32 on Solus 4, Epiphany 188.8.131.52 and WebKitGTK+ 2.24.1; On X11 (sadly)