The .. operator is used for joining strings together. It's present in Peter's example because it's combining the two strings "file://" and the result of MapPath(). You're not trying to join the two arguments of openfileexternal() that way—that would be nonsensical, so you'll get an error from not having a string segment before that operator.
I recommend keeping Programming in Lua handy. I have a hardcopy of the book on my desk at all times, but the first edition is available for free on the web:
https://www.lua.org/pil/contents.html#3Section 3.4 deals with the string concatenation operator.
edit: Also, hi Peter! Good to see you in here!