URL Path 后缀名

NSString对象调用pathExtension属性，即可获得后缀名。

<NSPathUtilities.h>
@property (readonly, copy) NSString *pathExtension;

---

例如：

NSString *suffix = [sourceString.pathExtension lowercaseString];
if ([suffix isEqualToString:@"png"] ||
    [suffix isEqualToString:@"jpg"] ||
    [suffix isEqualToString:@"gif"] ||
    [suffix isEqualToString:@"mp4"] ||
    [suffix isEqualToString:@"mov"] ||
    [suffix isEqualToString:@"com"] ||
    [suffix isEqualToString:@"html"] ) {
      [self doSomething];
    }