...but it's not a disk image. It's a disk image inside of a bzipped archive. You must first remove the bzip compression, and only then will you have a .dmg file. The only way to remove bzip2 compression is to uncompress it. Removing the file extension does not accomplish this.
Think of it as "reading right-to-left." If you have a file named aaa.dmg.bz2.sit.zip, you need to work from the right to the left -- first, you need to "unzip" the file (.zip), then you need to "unstuff" the file (.sit), then you need to un-b-zip the file (.bz2) then you will be left with a .dmg file. There's more than one compression to this file, as indicated by the multiple extensions.
Removing the extension, hidden or not, will not make the archive any easier to open. The most you will accomplish is fooling the computer into thinking that it's a straight .dmg file, but when it tries to open it, it will fail -- because it's still bzipped. The process that mounts/opens DMGs does not understand bzip compression, and, as such, will not be able to open the .dmg file -- because it's not a .dmg file. It's a bzipped .dmg file.
I fully understand your logic, Nato, but it will just not work in this situation. The bzip compression must be removed before the file will become a .dmg file.
You must uncompress the bzip file first -- there is absolutely no way around it. StuffIt Expander is the way to go with this, but it's not the only way to go.
Do this:
1) Open Terminal.
2) Type "bzip2 -d " exactly like that, without the quotes. Note the trailing space at the end.
3) Drag-and-drop the .dmg.bz2 file into the Terminal window. The path should be filled in automatically.
4) Hit enter. The file should be decompressed, leaving you with a straight .dmg file.
5) Double-click the .dmg file to mount it.