OK, Mr./Ms. Unregistered who checked this thread four months ago:

Does this happen with ALL files with suffixes .sit, .img, etc., or just some of them? Is there an error message? If so, what is it? Do you have problems with .sit, .img., etc. that you download from other sources? And you are certain that you are not trying to mount a .dmg in 9.x?

Have you ever wondered why there were so many different sizes for the same file or installer? There is junk out there: whether it is ignorance or malice is a matter we need not go into. So some of these files you download are likely going to be trash.

Now, for those that are not trash, dragging to the icon of Expander is going to work because the file was stuffed with StuffIt in the first place. If the file has a generic icon and in get info. (command+i) it is called a Limewire document, then double-clicking will just take you back to Limewire. An .img file is designed to work with DiskCopy. A .smi is self-mounting; so is a .sea. Both Disk Copy and StuffIt have been used without problems by millions of Macintosh users since they were introduced.

If worse comes to worse, get HexEdit and open the file and see what the header says.