[cs631apue] how to check the file extension

[bzhang41]

Dear Prof.

Last class I asked you about how to check the file extension for the 
"Content-Type" header, and you suggested a library function which will 
return the file extension. But I accidentally delete my class note when 
I try "1>mynote" on the terminal, I wanted to try "1>>mynote".
So I lost my notes and forgot what that function is, can you tell me 
again ?
I googled quite a while, but I cannot find the function you tole me.

Thanks
Bo