dowloaded files or files from my dropbox- it keeps trying to open them with skype!

When i try to open dowloaded files or files from my dropbox- it keeps trying to open them with skype. Even if i go to applications and select all applications and select preview to open it, it still goes back to trying to open in skype. I can see that someone else was having this problem and has been told that is not possible but I am also experiencing the same problem. Does anyone know why this is and how to fix it?