Does PHP `Imagick::writeImageFile` support two arguments and if not how to stipulate format?

Question

The official PHP documentation is a little imprecise whether Imagick::writeImageFile only supports a single file stream as parameter or also supports an additional second parameter to stipulate the image format.

If the second parameter is indeed not (officially) supported how do I stipulate the image format to use? Note, that streams opposed to file names have no intrinsic file extension. For example, in my specific use case most streams are memory buffers obtained by fopen('php://memory').