When generic icons are displayed, this is either due to your computer not having the appropriate application installed, or the Finder has become corrupted and you need to rebuild your desktop. Changing the file extensions updates the information in the header. Many file types, such as TEXT or jpeg or mov, may be opened by multiple applications. When the OS doesn't find the application that created the document, it looks for other applications that can translate the file type. Occasionally the vendor makes additional converters available later.
Some programs are specific to one type of translation and others are multipurpose.
It not only decompresses files compressed with Stuffit, but also Compact Pro, Disk Doubler and Zipit, and it decodes many encryption formats as well. The Windows version is called Aladdin Expander. If you receive an e-mail message that has been forwarded multiple times that you can't read, it may be because the message now has multiple headers at the top of the file. You should clear the excess information away before you can use any program to translate the document. On machines whose OS supports drag and drop, you should be able to simply drop the file onto the application for translation.