Also, when the forums try to linkify a plain text link, "(" and ")" in the link must be escaped with a backslash or you get:
http://www.example.com/some file ( M2 ).mhtml
However, reddit's markdown handling doesn't require you to do that. Neither does stackoverflow's. They know the( and ) in this case are part of the URL, especially since there's no use of the []() syntax in this case.
But, even with the []() syntax, stackoverflow knows the ( and ) inside the URL itself are part of the URL and not a delimiter/opener/closer for the link syntax.
Stackoverflow's and Reddit's handling just seems to be more robust.
Stackoverflow uses the StackEdit editor that uses the pagedown markdown converter. They're both open source.