Shell > Script
ShellScript is a Script written for the Shell (They are similar to *.bat or .cmd files in Windows).
The MIME type is application/x-shellscript
They are text files which first line says the shell to use (like bash , dash, csh, tcsh). So the first line is
#!/bin/sh
Including this line, Debian knows this is a shell script (they don't need to finish with *.sh).