MFC Save As dialog

I'm writing MFC application and my application needs to have
a Save As dialog. I manage to do this with the following code (among other code):

Code:

char str[]="*.txt||";
	CFileDialog FDia(FALSE, "txt", "Filter",
		OFN_HIDEREADONLY | 	OFN_OVERWRITEPROMPT,str,NULL);

And this works fine.
If try to write something like this (this was my first choice):

Code:

char str[]="*.txt";
	CFileDialog FDia(FALSE, "txt", "FilterKoef",
		OFN_HIDEREADONLY | OFN_OVERWRITEPROMPT,str,NULL);

When user click Save button Save As dialog appears and in Save as type I have *.txt and couple of rubbish characters.
My question is why when using first solution everything works fine and what symbol "||" means? Why using "||" in str
fix problem?
Thanks for help!