Kentico upload a file using the API to a Page Type's File field

I have a page type (let's call it MyPageType) with a field PreviewImage that is of data type File.I am trying to use the Kentico APIs to upload files from the server to that PreviewImage field on the MyPageType page type. From what I can see it seems that I have to upload the file somewhere and then the uploaded file will have a GUID that I can use for the PreviewImage field.