There's probably a shortcut for this, but at the moment I go to the Media Browser, scroll to the bottom of my media to the tiny sliver of a blank space between the last file and the horizontal scroll bar, right click and select Import.
I've moused over the other buttons there and I can't seen another one just to import a file.
Just now I tried right-clicking around that pane to see where else brings up that right-click menu, and it's only the space above the "x of n items selected" text (to the right of the directory name) that also brings up this menu with the "Import" option.
My suggestion is that after the last item in the media list, there should be a blank row, and in the column where the coloured box is for each file, put a + button there to be clicked on to bring up the options to create a new item or import a file.
That way, the vertical scroll will always scroll far enough to show the whole empty row with this + button at the start of it, and it'll be easier to find this function.