On Mac OS X (LaunchServices and UTIs) to convert a file path to the file's MIME type using.

gistfile1.m

+ (NSString *)MIMETypeForFileAtPath:(NSString *)path defaultMIMEType:(NSString *)defaultType
{
    NSString *result = defaultType;
    NSString *extension = [path pathExtension];
    CFStringRef uti = UTTypeCreatePreferredIdentifierForTag(kUTTagClassFilenameExtension,
                                                            (CFStringRef)extension,
                                                            NULL);
    if (uti) {
        CFStringRef cfMIMEType = UTTypeCopyPreferredTagWithClass(uti, kUTTagClassMIMEType);
        if (cfMIMEType) {
            result = [[(NSString *)cfMIMEType copy] autorelease];
            CFRelease(cfMIMEType);
        }
        
        CFRelease(uti);
    }
    
    return result;
}