To actually change your extensions, you need to set your computer to show known file extensions. Otherwise you're just making the name myfile.ztd.zip.
There may be a tutorial on how to do this already somewhere, but what you do is in any Windows Explorer window, go to Tools and then Folder Options. Click the View tab and uncheck the box that says "Hide extensions for known file types." Hit Apply.
You'll now notice that all your files have extensions. Your zips will say .zip and your documents will say .doc.
THEN if you rename the file, and get rid of the .zip and put .ztd, you're actually changing the extension. The system will probably give you a warning asking if you're sure you want to do that.
Don't know about your other problem, but hopefully that helps.