Specifies that this property is the primary media for the content type. This media will be used as thumbnail for the content in the back office, and as the upload target property for the default file upload handler (when used in combination with the MediaUploadAttribute). Can only be set on IFileDescriptor property types. This attribute is only effective on content types with multiple file properties and does not need to be specified on content types with a single file property, as the single file property will be selected as the primary media property by default.
public sealed class PrimaryMediaAttribute: IContentPropertyAttribute