I'm not sure what you mean by opening files/folders.
You can use /run to open files and folders, and also web sites:
/run wordpad.exe readme.txt
From the documentation:
/run [-hnp] <filename> [parameters]
Runs the specified program with parameters.
The -h switch attempts to hide the window of the application being run.
The -n switch minimizes the window of the application being run.
The -p switch sets the working path to the path of the application being run.
You can enclose the filename or parameters in quotes if you need to. If you specify a non-executable file, mIRC tries to open it with the application associated with that file.