NSString+AnotherClassExtension.h

@interface NSString (AnotherClassExtension)

- (NSURL *)toURL;
- (NSURL *)toFileURL;
- (NSString *)toDocumentDirectoryPath;
- (NSURL *)toDocumentDirectoryFileURL;
- (NSString *)toCachePath;
- (NSURL *)toCacheFileURL;

- (UIImage* *)toImage;

@end

NSString+AnotherClassExtension.m

@implementation NSString (AnotherClassExtension)

- (NSURL *)toURL {
    return [NSURL URLWithString:self];
}

- (NSURL *)toFileURL {
    return [NSURL fileURLWithPath:self];
}

- (NSString *)toDocumentDirectoryPath {
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *dir = paths[0];
    return [dir stringByAppendingPathComponent:self];
}

- (NSURL *)toDocumentDirectoryFileURL {
    return self.toDocumentDirectoryPath.toFileURL;
}

- (NSString *)toCachePath {
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES);
    NSString *dir = paths[0];
    return [dir stringByAppendingPathComponent:self];
}

- (NSURL *)toCacheFileURL {
    return self.toCachePath.toURL;
}

- (UIImage* *)toImage {
    return [UIImage imageNamed:self];
}

@end