Some Web servers have trouble handling plain .dmg files, because they don’t know what kind of file they are. When people download the .dmg file, it can open in the Web browser instead of mounting automatically. Or, with a bzip2-compressed image, Safari will change the file extension from .dmg to .dmg.bz2, which causes the OS to think the file is damaged. In order to prevent these problems, make sure that your Web server uses the correct MIME type for .dmg files. If you administer an Apache-based server, you can ensure this by adding this line:
AddType application/x-apple-diskimage .dmg
to your .htaccess file.