Not sure if the message file type is exclusive but from what I know the /f switch is for .msg and .oss files. Here are the Outlook command line switches for 2010 but probably whatever version you’re using.
That said, if .eml is associated with Outlook, you can just put the .eml filename as the process FileName in the Start Process activity and leave the Arguments field blank. That’s how I usually use it if I’m forwarding an email I’ve just saved or whatever.