If I understand correctly, the method for an open panel looks like:
<BLOCKQUOTE><font size="1"face="Geneva, Verdana, Arial">code:</font><HR><pre><font size=1 face=courier>
- (void)beginSheetForDirectory

NSString *)path
file

NSString * name)
types
NSArray *)types
modalForWindow

NSWindow *)docWindow
modalDelegate

id)delegate
didEndSelector

SEL)didEndSelector
contextInfo

void *)contextInfo
</font>[/code]
types is where you can specify the file types to be allowed for opening. This can be a array of strings generated by you, or it can be an automatically generated one (for example, "types:[NSImage imageFileTypes]".
(For more info, see Cocoa Developer Documentation -> File, Resource and Process Management -> Application File Management -> NSOpenPanel (or NSSavePanel))