How do I tell Nautilus to use application based on file content instead of extension

I need / like Nautilus to determine the file type based on the content of the file (similar to the file command) and not on the extension and base the decision which program to use (when the file is double-clicked) to open the file on that.

E.g. I have a picture called screenshot.txt (call me an idiot but that is how it is) and nautilus should open it with an image-viewer (e.g. EOG) and not with a text editor (e.g. gedit). If the picture was simply called screenshot (without an extension), nautilus behaves correctly and uses an imageviewer.

How do I achieve this?

Ubuntu 10.04 and 12.04, basically standard installs, both up-to-date