The expected number of bytes that need to be received to finish loading the image.
This value is not necessarily equal to the expected size of the image in bytes, as the bytes required to load the image may be compressed.
This value will be null if the number is not known in advance.
When this value is null, the chunk event may still be useful as an indication that data is loading (and how much), but it cannot represent a loading completion percentage.
final int expectedTotalBytes