Shotwell only checks (or at least that is how it works here) for filename/extensions to see if it supports an image format, this is really bad, because most modern image viewers don't do that, and if you download some image which doesn't have an extension it won't open in shotwell, you need to rename it manually if you know why it failed. For comparison EOG (Eye of GNOME) works fine with  'image.png' or 'image' (both same files, just second without extension written), shotwell fails on second with "Shotwell
Shotwell does not support the file format of /path/to/file"