base64Contentskotlin.StringBase64 encoded string of file contents. Typically this means reading the bytes or string content of a file and then converting that to a base64 encoded string. For examples of how to do this see https://www.mailslurp.com/guides/base64-file-uploads/
contentTypekotlin.StringOptional contentType for file. For instance application/pdf[optional]
filenamekotlin.StringOptional filename to save upload with. Will be the name that is shown in email clients[optional]