I just tried the plugin for the first time and I ran into what appears to be the same problem. I select an image for download and a file downloads with no file type suffix. I added .jpg and was able to open the image. Tested with Firefox on OSX Mavericks.
I proceeded to test with Safari. Interestingly the 1st 3 images in the gallery were already selected when the page loaded. When I selected an image and clicked the download button, a blank browser window opened, the file downloaded with a .jpg suffix and the file opened automatically in photoshop. Tried again and the same results.
Also surprisingly, downloads are not zipped even though I have settings configured to zip automatically.
Site is under development so I can only supply a link privately by email.