Nope, i have testit it with both settings (file extensions on and off) both load the script file as it should be.
I also registered the filetype .mrc to a custom app to hide the extension, it's not related to that bug.
Tested on Win7 x86 Prof. with mIRC 7.14
Perhaps you didn't reopen the file load dialog after changing the setting or something, but I can confirm that HaleyJ's bug only occurs when "Hide extensions of known file types" is enabled. This bug and the one that starbucks_mafia linked to are the same bug.
(Edit: It's possible this bug only affects some versions of Windows 7. I'm running 64bit Ultimate.)
