Ignore the chrome warning. There is no standard MIME type for OTF fonts.
font/opentype may silence the warning, but that doesn't make it the "right" thing to do.
Arguably, you're better off making one up, e.g. with "application/x-opentype" because at least "application" is a registered content type, while "font" is not.
Update: OTF remains a problem, but WOFF grew an IANA MIME type of application/font-woff in January 2013.
Update 2: OTF has grown a MIME type: application/font-sfnt In March 2013. This type also applies to .ttf