How to change the default program for a chosen file type system-wide in kde or xfce?

Question

I would like to have files of a chosen type (by their extension) to open with some specific programs. Which configuration files do I need to edit?
I've tried some in /usr/share/appplications/default.list but it doesn't work.

Solution

Have a look at the file $HOME/.local/share/applications/defaults.list

There is a section [Default Applications] to specify the programs for particular mime types. You can add for example:

application/x-debian-package=gdebi.desktop

The .desktop files can be found in /usr/share/applications/ or you can create your own files under $HOME/.local/share/applications/. To find out the mime type of the files, run

file --mime-type <some_file>

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Alternatively, in KDE one can accomplish the same in GUI in File Associations inside System Settings.

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To register a MIME type for an extension see freedesktop.org - AddingMIMETutor and stackoverflow - Register file extensions / mime types in Linux.

Comments

[igor012]

I have modified the mime type `MimeType=image/x-exr;image/tiff;image/png;image/jpeg;image/‌​dpx;' in the .desktop file.

[Anatoly Agapito]

On my version Fedora release 18 (Spherical Cow) running xfce, the file was named mimeapps.list, not defaults.list. JTYSK

[appas]

This can be used to set the application based on MIME type, but what about file extension (as per the original question)? For instance, Processing sketches have the MIME type text/plain ("plain text document"), but they can be told apart by their .pde extension. I'd like to set them to open in Processing, but have not been able to do so without affecting .txt's and others as well.

[Marco]

@appas First you register the file extension to a MIME type. Then you assign an application to the MIME type as depicted in this answer.

[Edward]

You can also find the custom "Open with" .desktop (Desktop Entry) files in this local applications folder - they're automatically created here. Some of the options inside the file can be edited with the "File Properties" GUI from the file manager.