After debugging I have found the Content Type Ids for Image, Audio and Video in the assets library. These content type ids are not present in SPBuiltInContentTypeId.
public class SPBuiltInContentTypeIdExtension
{
public static SPContentTypeId Video =
new SPContentTypeId
("0x0101009148F5A04DDD49CBA7127AADA5FB792B00291D173ECE694D56B19D111489C4369D");
public static SPContentTypeId Audio =
new SPContentTypeId
("0x0101009148F5A04DDD49CBA7127AADA5FB792B006973ACD696DC4858A76371B2FB2F439A");
public static SPContentTypeId Image =
new SPContentTypeId
("0x0101009148F5A04DDD49CBA7127AADA5FB792B00AADE34325A8B49CDA8BB4DB53328F214");
}
These three asset content types inherit from Document CT (“0x0101”) and have “0x0101009148F5A04DDD49CBA7127AADA5FB792B” in common, which is the content type id for multimedia content type. So if you want to check if it is a multimedia, use this id. You can see the content type id when you go to Site Actions - Site Settings - Content Types. Click on a content type you interested in. In the address bar of your browser find ?ctype=.