A .mint file only contains the information and sources to download and install software. If you only download a .mint file, you still have to open it with mintinstall. Mint uses .deb files for software installation with dpkg via apt/synaptic.(same as Debian/Ubuntu). For example, if you want to install Firefox with mintinstall, it uses the instructions in the .mint file to know where to get the firefox.deb and all of the other packages required to make it work. If you want to download the firefox package to your hard drive, then you will have to download the firefox.deb package here:
http://packages.linuxmint.com/ I hope I didn't confuse you more

"The only thing necessary for the triumph of evil is for good men to do nothing." Edmund Burke