Files can be entered into the Windows Command Prompt as standalone commands - "C:/full/path/file.txt", for example, will open the targeted text file.
.exe files can also be executed this way - any arguments that may be required can be passed normally, for example:
"C:\WINDOWS\system32\notepad.exe" "C:\full\path\file.txt" will run Notepad, opening the file specified.
Keep in mind that the best way to open and execute files is through the "Run Application" Action, or through the "Cmd Session" group of Actions. The "Run DOS Command" Action, while it may work for simple commands, is not meant for this kind of interaction, but rather for pure DOS commands, for example:
This is an important distinction when trying to run, for example, .exe files with arguments; the "Run DOS Command" Action will fail, while the above recommended Actions will succeed.