ARC and CFRelease?

You can’t release mimeType because you don’t own it. You didn’t transfer ownership with the __bridge cast.

You should be releasing uti since you have created it.

You should also release extension since you created it as well, but that will likely cause issues with ext. Instead, transfer ownership to ext.

I’d suggest the following:

+ (NSString *) fileExtensionForMimeType:(NSString *)type {
    CFStringRef mimeType = (__bridge CFStringRef)type;
    CFStringRef uti = UTTypeCreatePreferredIdentifierForTag(kUTTagClassMIMEType, mimeType, NULL);
    CFStringRef extension = UTTypeCopyPreferredTagWithClass(uti, kUTTagClassFilenameExtension);

    NSString *ext = (__bridge_transfer NSString *)extension;

    // CFRelease(mimeType); // not owned
    if (uti) CFRelease(uti);
    // CFRelease(extension); // ownership was transferred

    return ext;
}