Cancel button does not return a callback

[guneyozsan]

Hello.

This is actually a feature request. Cancel button currently does not return a callback. It would be great to have one.

Receiving a callback like NoFileSelected when cancel button is clicked can be helpful in some scenarios. Sometimes complex upload objects are prepared in advance and it is good to know if they could disposed safely. Otherwise the code for WebGL and PC diverses with introduced complexity which would otherwise easily prevented. This is especially important since debugging for Unity WebGL is like hell.