Question

up.removeFile(file) works only if the upload is not in progress.

Is this a bug, or exists some other function I missed to call?

Answer 1

This way I could make it work.

Every added files has it own cancel function:

var status_before = file.status;
up.removeFile(file);                
if(up.state == plupload.STARTED && status_before == plupload.UPLOADING)
{
  up.stop();
  up.start();
}