From Xojo Documentation

Property (As String )
aFileType.MacCreator = newStringValue
StringValue = aFileType.MacCreator

Supported for all project types and targets.

The four-byte string used to represent the application that owns the file in the Mac OS.

Sample Code

The following defines the jpeg file type in code.

Dim jpegType As New FileType
jpegType.Name = "image/jpeg"
jpegType.MacType = "JPEG"
jpegType.MacCreator = "prvw"
jpegType.Extensions = "jpg;jpeg"