@Subharo Bhikkhu,
I got what you are trying to explain... The problem is that you are wrong...
And i will try to explain you why...

1. 'exo-preferred-applications' is part of XFCE4 and is installed because by default Ubuntu installs 'xfce4-terminal' that depends on the 'exo-utils' package. Actually 'exo-preferred-applications' is part of 'exo-utils' package. It is intended to be used in XFCE only

2. Every application should be XDG compliant and should use its specifications. According to these specs an app may install a .desktop file and claim handling of a specific mime type by adding to its  .desktop file an entry named 'MimeType' (http://standards.freedesktop.org/desktop-entry-spec/desktop-entry-spec-1.0.html and other resources)

3. In this particular bug/case  'exo-utils' adds the /usr/share/applications/exo-file-manager.desktop which claims the file:// scheme to be handled by "exo-open --launch FileManager %u":

==========================
[Desktop Entry]
Version=1.0
Type=Application
Exec=exo-open --launch FileManager %u
MimeType=x-scheme-handler/file;x-scheme-handler/trash;
==========================

'--launch FileManager' here means use the file manager chosen by 'exo-preferred-applications'... so if you have chosen Nautilus there, practically files will be started by this command:

nautilus file:///example_text_file.txt

4. Nautilus cannot open files using the 'file://" schema.... It opens only folders when using this schema... 

5. Thunar on the contrary CAN open files this way... (like thunar file:///.... )
That is why you can open files using Thunar and cannot by using Nautilus....

6. Now most gnome applications use "gnome-open" internally to detect which program to use to open a file. It is part of the 'libgnome2-0' package.... The problem here in this bug is that gnome-open always complies to XDG specs and uses the /usr/share/applications/exo-file-manager.desktop to open files (this .desktop file had already claimed that it will do that). The fail comes because of Nautilus (see number 4)

7. To fix all this mess the /usr/share/applications/exo-file-manager.desktop needs to be fixed as the 'MimeType=' entry, should be rewritten as: 'X-XFCE-MimeType=' which means: "claim to open all files using 'xdg-open --launch FileManager' ONLY if we are in XFCE"

So in a few words... the bug exists because when some app tries to open the file /tmp/test.txt for example, the following happens when in GNOME:

A: When nautilus is selected in exo-preferred-applications

gnome-open /tmp/test.txt -> xdg-open --launch FileManager /tmp/test.txt -> nautilus /tmp/test.txt -> nautilus is not able to open files -> error 'Location is not a folder'

B: When Thunar is selected in exo-preferred-applications

gnome-open /tmp/test.txt -> xdg-open --launch FileManager /tmp/test.txt -> thunar /tmp/test.txt -> file is opened ok


When you are in XFCE things work ok without this bug...



Anyway this bug should be fixed as i described in my post #14 (and as i already said the fixed exo package is in my PPA and have been tested to work ok in GNOME and XFCE)

I am sorry if my explanations are a kinda unclear or too simplified but i cannot explain it better...

So there is no need the bug to be renamed etc... all that is needed is the fixed package to be tested and uploaded to ubuntu's updates repos...