I need to create NSString for file name based on screen width, if the file is fixed to be a PNG file, I can do:

```
[filename stringByReplacingOccurrencesOfString:@".png" withString:@"_320.png"]

```

but the file may not be PNG file, so I need a more general way to add a "\_320" to file name regardless of its extension, how do I do this?

Thanks a lot!