Since version 6.5 you prefix the part of the expression which specifies the exclusion group, e.g.
With this syntax you can then also add additional criteria, e.g. to search for all files with the word
Window excluding PDF files you can enter:
Or, to search for any image file with the word
New York you can type:
There's a few more nice DOS Expression features since 6.5 so be sure you check out the help file:
Help: DOS Expressions
Note: In version 6.2 (or earlier) you could exclude multiple types by clicking the Not check box next to the File name: field and then specifying the types to exclude, e.g. the DOS Expression
This was changed with version 6.5 to allow more flexibility with DOS Expressions.