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

Including this line, Debian knows this is a shell script (they don't need to finish with *.[:Sh:sh]).