How to change default program for files ending in one extension

This answer is adapted from Ondra Žižka's answer here: How do I set the default program?. Please upvote there as well if this helps you.

To change the default program for files with a specific file type, a mime type needs to be created for each file extension. This is because Ubuntu tracks this indirectly:

• file extension -> MIME types
• MIME type -> application(s) to open
• Applications -> list of compatible MIME types

For Ubuntu (18.x), the following applies:

file extension -> MIME types

This is determined from files in /usr/share/mime/packages/. E.g.:

cat << EOF | sudo tee /usr/share/mime/packages/staruml.xml
<?xml version="1.0" encoding="UTF-8"?>
<mime-info xmlns='http://www.freedesktop.org/standards/shared-mime-info'>
  <mime-type type="application/staruml-project">
    <comment>StarUML project</comment>
    <glob pattern="*.MDJ"/>
    <glob pattern="*.mdj"/>
  </mime-type>
</mime-info>
EOF

Other xml files are generated from these and placed in /usr/share/mime/. Sometimes these files may be found online, especially for open source software packaged for Ubuntu. For example, the mime files for KiCad can be found in the KiCad source code repository: https://gitlab.com/kicad/code/kicad/-/tree/master/resources/linux/mime

MIME type -> application(s) to open

This is driven by ~/.config/mimeapps.list or possibly ~/.local/share/applications/mimeapps.list. This should override /usr/share/applications/mimeapps.list. For example:

echo 'application/staruml=staruml.desktop' | tee -a ~/.local/share/applications/mimeapps.list

Or, to change for all users:

echo 'application/staruml=staruml.desktop' | sudo tee -a /usr/share/applications/mimeapps.list

Applications -> list of compatible MIME types

This is driven by the .desktop files in /usr/share/applications/. That file can contain a MimeType= entry with semicolon-separated list of MIME types. For instance, the EOG app has MimeType=image/bmp;image/gif;image/jpeg;image/jpg;....

cat << EOF | sudo tee /usr/share/applications/staruml.desktop
[Desktop Entry]
Name=StarUML
Comment=StarUML
GenericName=StarUML
Exec=/sw/prog/StarUML/StarUML-3.1.0-x86_64.AppImage %U
Icon=/sw/prog/StarUML/staruml.png
Type=Application
StartupNotify=false
Categories=Development;Programming;Modeling;UML
MimeType=application/staruml-project;
EOF

This makes it a full circle. The changes take effect after running

sudo update-mime-database /usr/share/mime

and, for icons,

sudo gtk-update-icon-cache /usr/share/icons/gnome -f

Or, you may need to logout/login.

Then, mimetypes for a given file extension can be checked using mimetype:

$ mimetype .mdj
.mdj:  application/staruml-project

References:

• How do I set the default program?
• https://coderwall.com/p/qjda2q/create-new-mime-type-and-assign-an-icon-to-it-in-ubuntu
• https://help.ubuntu.com/community/AddingMimeTypes
• How to assign (set) a MIME type to a file?

Credit: Ondra Žižka (Adapted)