I changed the default application association for a mimetype that was wrong just editing the file .local/share/applications/mimeapps.list.
Just doing this was not sufficient to change also the default icon, so how can i set that the default icon for this type of file should be a .png in my home? (I can move it anywhere)
Right clicking on one of them and change the icon just change the one of that specific file, not for all the others with the same mimetype.