Why do my files open in the wrong application?

Every file type has a default application registered with the operating system, and opening a file consults that registration rather than anything about the file itself. Installers frequently claim associations for types they can handle, sometimes without asking clearly, which is how a newly installed program ends up owning file types you never intended.

The fix is to reassign the association, and both major desktop systems offer two routes. The per-file route is to open the context menu, choose to open with a specific application, and select the option to always use it for this type. The system-wide route is a settings page listing file types and their defaults, which is more efficient when several types moved at once.

A few situations complicate this.

• Some file extensions are ambiguous by nature. Plain text, comma separated data and various image formats are all legitimately handled by several applications, and which one is correct depends on what you intend to do rather than on the file.
• Uninstalling an application does not always release its associations cleanly, leaving types pointing at something no longer present. Reassigning explicitly resolves this.
• Web links to local applications work through a different mechanism and are configured separately, so fixing a file association does not change what happens when a link tries to open an application.

If associations keep reverting, an application is reclaiming them at launch or after updates. Most such applications have a setting controlling this, usually presented as checking whether it is the default on startup, and disabling that stops the behaviour permanently.