GitHub detects .d source as Makefile?

[XavierAP via Digitalmars-d-learn]

So I have put my first code even on GitHub (comments welcome :)) 
and GitHub seems to detect the wrong language, even if I'm not 
familiar with this GH feature.

https://github.com/XavierAP/etc/tree/master/heatsim

The repository itself ("etc") is flagged as been written in 
Makefile? Right now I have only two source files (and a couple of 
images and a pdf). If I look at syntax highlighting online, one 
of them main.d seems highlighted in D ok. But the other one 
heatsim.d is not correctly highlighted.

https://github.com/XavierAP/etc/blob/master/heatsim/src/heatsim.d
https://github.com/XavierAP/etc/blob/master/heatsim/src/main.d

Is this a known issue with D on GitHub? Should I report it I 
guess?
How smart is GH that it doesn't look at the file extension? What 
happened?