Pixiq

I found and interesting thing today while in the Filmstrip. With one image selected in Loupe view (or Grid) using the shortcut Shift Command Down Arrow (Shift Control Down Arrow on PC) will select an additional 6 images. This can be repeated to add another 6 and so on. Using Shift Command Up Arrow (Shift Control Up Arrow on PC) will remove 6 images from the selection.
If you start with an image on the middle, then using Shift Command Up Arrow (Shift Control Up Arrow on PC) will select images to the left of the current image, while Shift Command Down Arrow (Shift Control Down Arrow on PC) will deselect them.

Update: Troy Gaul tell me the number (6 in my case) is based on the number of thumbnails in the grid row for grid view (obviously not visible in Loupe view), so having smaller thumbnails or a wider window will increase this number.

For single image additions to the selection, use Shift Command Left Arrow (Shift Control Left Arrow on PC) to add to the right and Shift Command Right Arrow (Shift Control Right Arrow on PC) to the left. The opposite arrow will deselect (i.e. if you use the left arrow to select, the right arrow shortcut will deselect).