On double click, Nautilus opens the file with the wrong application

Observed behaviour
Assume I have a file called screenshot.png and double click it; Nautilus will start the image viewer. If I omit the extension, Nautilus is clever enough to figure it out and open it with the image viewer. If I'm however stupid enough to rename the file to screenshot.txt, Nautilus decides that it's no longer an image and opens with a texteditor (or attempts to).

Expected behaviour
Nautilus should use the file content to determine the type of file and the associated application instead of extensions.

Reported for Ubuntu 12.04; also observed with an up-to-date 10.04 and also reported to be the case on 13.04 (see https://answers.launchpad.net/ubuntu/+source/nautilus/+question/214355)

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: nautilus 1:3.4.2-0ubuntu5
ProcVersionSignature: Ubuntu 3.2.0-33.52-generic 3.2.31
Uname: Linux 3.2.0-33-generic x86_64
ApportVersion: 2.0.1-0ubuntu15
Architecture: amd64
Date: Sun Nov 18 13:58:04 2012
ExecutablePath: /usr/bin/nautilus
GsettingsChanges: org.gnome.nautilus.window-state geometry '1178x772+740+399'
InstallationMedia: Ubuntu 12.04.1 LTS "Precise Pangolin" - Release amd64 (20120823.1)
MarkForUpload: True
ProcEnviron:
 LANGUAGE=en_ZA:en
 PATH=(custom, no user)
 LANG=en_ZA.UTF-8
 SHELL=/bin/bash
SourcePackage: nautilus
UpgradeStatus: No upgrade log present (probably fresh install)