for k, vv := range f.File {
			vv2 := make([]*multipart.FileHeader, len(vv))
			for i, v := range vv {
				vv2[i] = cloneMultipartFileHeader(v)
			}
			m[k] = vv2
		}
		f2.File = m
	}
	return f2
}

// cloneMultipartFileHeader should be an internal detail,
// but widely used packages access it using linkname.
// Notable members of the hall of shame include: