I would expect xdg-open command to use the same application that opens when I double-click the file in the default file manager, but this is not always true.
For example my DE is XFCE, my file manager is Thunar and my default picture viewer is Ristretto. However, xdg-open example.png opens the example PNG file in Pinta. Why?
xdg-open is a desktop-independent tool for configuring the default applications of a user. Many applications invoke the xdg-open command internally. Inside a desktop environment (like GNOME, KDE, or Xfce), xdg-open simply passes the arguments to those desktop environment's file-opener application (eg. gvfs-open, kde-open, or exo-open). which means that the associations are left up to the desktop environment. When no desktop environment is detected (for example when one runs a standalone window manager like eg. Openbox), xdg-open will use its own configuration files.
from archwiki
specific to your question, you could try this to set the default application associated with the png file:
xdg-mime default <ristretto.desktop> image/png
you need find out what exactly the desktop file name of Ristretto. afterwards, you could check it with this:
xdg-mime query default image/png
xdg-mime query default image/png returns wine-extension-png.desktop :-)
@Ivan That happens probably, because you didn't provide the right filetype when asking for the default application. Important is how xdg calls that specific file-type. This can easily be found out by running
xdg-mime query filetype example.png
As an example, if I do this with a specific png-file on my computer it returns image/x-apple-ios-png. Once you have the actual file type figured out, you can run
xdg-mime query default <new filetype>
and get the application that is used by xdg for this specific file type. In case no application is defined for this filetype, xdg-open checks if the file /etc/debian_version is available, if this is the case, it will launch run-mailcap, which is very similar to xdg-open and tries to find a program to open the file as well.
Only if this fails too, xdg-open will ask the user to define a program to open the file.
xdg-mime query filetype example.png to check the filetype of a specific file, because simply assuming that a .png file also falls into the category image/png is not always correct. Looking at Ivan's comments, I think that his issue was not resolved yet.