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?
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();
}