Try taking one letter off the extension of the file, or adding 1. So for a .jpg file, you'd cut down to .jp or make it .jpge. Something like that.
Then to view, just explore, change it back and you're done. Not really convenient, but it doesn't accidentally get opened that way.
Also, on your computer, you can associate a new extension with a viewer so that on your computer, there's no need to change the extension.