If you receive File in Use. The action can't be completed because the file is open in another program even if file is closed in another program, see this.

If you cannot associate file extension to application & see The program you selected cannot be associated with this file type error, see this fix.

Windows makes it easy for you to open up these files by setting up default programs for each file type -- images files, for example, will open by default in the Windows 10 Photos app.