There must be some error in your extension mapping. I have seen a similar problem with Gecko-based browsers, but never before with Safari. I am not familiar with the *.dms extension, so I don't know what may have caused the problem. However, all you really have to do is to change the extension to the correct one.
Or! Write a new folder action to auto rename .DMS files to .DMG files! Okay so that's probably not the slickest idea in the world but could be a temp fix...
well i did a google search on this issue, and found no answers just questions on how to fix this. therefore when i found the answer i decided to post it 😉