Mackail-Smith has scored 27 goals in League One this season and qualifies for Scotland through his grandmother.
He said: "I am absolutely over the moon and my gran will be the proudest gran in the world.
"She has followed my career since I was a kid and always hoped - as I did - that I would one day play for Scotland.
"I knew the Scotland scouts had been watching me this year but I certainly never imagined that I would be playing Brazil.
"The plan was to kick back on the sofa and catch it on the TV. I've had a great year and this caps it off."
Source: PA
Source: PA