I just did some research on IANA official list. I believe the answer given here 'font/xxx' is incorrect as there is no 'font' type in the MIME standard.
Based on the RFCs and IANA, this appears to be the current state of the play as at May 2013:
These three are official and assigned by IANA:
These are not not official/assigned, and so must use the 'x-' syntax:
The application/font-woff appears new and maybe only official since Jan 2013. So "application/x-font-woff" might be safer/more compatible in the short term.