You can achieve this pretty easily with the help of NSPredicate, like so:
NSString *bundleRoot = [[NSBundle mainBundle] bundlePath];
NSFileManager *fm = [NSFileManager defaultManager];
NSArray *dirContents = [fm contentsOfDirectoryAtPath:bundleRoot error:nil];
NSPredicate *fltr = [NSPredicate predicateWithFormat:@"self ENDSWITH '.jpg'"];
NSArray *onlyJPGs = [dirContents filteredArrayUsingPredicate:fltr];
If you need to do it with NSURL instead it looks like this:
NSURL *bundleRoot = [[NSBundle mainBundle] bundleURL];
NSArray * dirContents =
[fm contentsOfDirectoryAtURL:bundleRoot
includingPropertiesForKeys:@[]
options:NSDirectoryEnumerationSkipsHiddenFiles
error:nil];
NSPredicate * fltr = [NSPredicate predicateWithFormat:@"pathExtension='jpg'"];
NSArray * onlyJPGs = [dirContents filteredArrayUsingPredicate:fltr];