The Mixxx installer is packed into an WiX Burn-based EXE installer, as such dark.exe from the WiX toolset is needed to retrieve the actual MSI installer.
The MSI installer itself ships with the debug symbols for Mixxx which are about 190 MB in size. The file mixxx.mst in this folder contains a Windows Installer Transform to allow unpacking of all MSI files without the debug symbol file. Of course, alternatively, one can just unpack the file and delete it afterwards but that would theoretically require the user to keep at least 190 MB of free space available for things the installer will just delete after installation.