No, I didn't launch NeoOffice. It launched on its own. The disk .img contained Skype.
Then repeat what I said to do, but do it to a .dmg file. (You said disk .img, but I assume you meant .dmg.) Somehow, your system has been told that NeoOffice is the app to use to open .dmg files. Normally, they should be opened with DiskImageMounter.app (also found in /System/Library/CoreServices).
Your system maintains, for each user, a Launch Services database that keeps track of what app should normally handle various things: files, URLs, etc. The database bases its decision for opening files on filename extension. (It used to also consider file type and creator, but that got dropped many OS releases ago.)
When you Get Info on a file, select a different app from the "Open with:" menu, and click "Change All", what you're doing is updating the launch services database. (If you don't "Change All", and you have write permission to the file, the file is modified to say that Launch Services should be ignored for this file. It should be opened with the indicated app no matter what Launch Services says.)
The same thing happens in apps that let you configure which app handles various types of files and URLs. They're actually directly manipulating Launch Services (unless the app predates Launch Services and had to implement its own equivalent). Configure it in any app, and all apps will honor it.
I sounds like you've told your browser to expand .zip files as they get downloaded (a setting I strongly discourage as unsafe), it unzipped the file to a .dmg file, and for some reason tried to open that. Launch Services told it that the app to do that is NeoOffice, so NeoOffice is what got launched.
Stuffit Expander has an option to continue expanding as many levels deep as it can. If SE was used to expand the .zip file, and you have this option set, it would have tried to open the .dmg file, using Launch Services to do so.
I would recommend:
- Browsers should not expand or open downloaded files. (In Safari, for example, turn off the "Open 'safe' files after download" option under Preferences→General.) So-called "safe" files are not always safe.
- .zip files should be set to always open with Archive Utility.app. (You can still manually use Stuffit Expander on the rare files that Archive Utility won't handle.)
- .dmg files should be set to always open with DiskImageMounter.app.