If I have a .mrc file i.e test.mrc and I try and load it from within the script editor it won't work, unless I manually append .mrc in the windows explorer window that opens.

Steps to replicate:

1. Create a .mrc file in the folder that mIRC keeps your scripts.
2. Press alt + r to open the script editor
3. Click on file and select load
4. Navigate to the .mrc file and click open

One cannot open the file. I used to be able to open the file on previous versions without any problem.

I am using windows 7 and the latest version of mIRC.

No problems here, though I am using Windows XP rather than Windows 7, so that might be the difference.

I seem to recall another report of this on Win7.
Do you happen to have the script file already loaded and are you (accidentally) trying to load it again?

This is almost certainly linked to this bug, in which case it should be fixed for the next release.

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

And about HaleyJs problem
I can't confirm or reproduce it on Win7 or XP

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.)