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

How to Stop Photoshop Files From Opening in Other Programs. When your operating system forgets to associate the PSD files you build in Adobe Photoshop with the application you use to create them ...

File name extensions are how OS X recognizes file types, and changing the extension is one way to associate a file with a different program. Sometimes, however, changing the file extension does ...