Hello,
Is there a way to tell dolphin to open files according to their file type (or content) and not their file names?

I have a file that ends, for example, by .jpg. It could be any other ending, they're just file names.

When I click on that file, it wants to open it with an image viewer. That file is not an image.
If I remove the .jpg part of its name, the file opens with the correct application.

thanks.

This is correct behavior because applications are associated to certain file extensions. You have these options in system settings Programs / Applications. You don't want to mess with it. You need to use some application, for example Krename, GPRename, pyRenamer to change file names and give proper extensions.