MIMEBoundary is the string that identifies the boundary between the parts of the multipart form. It is an attribute of the ContentType header for the entire form. 

When the MIME attachment handler is processing a received form, this property is not available until after the application has called the ProcessMultiPartForm method. When the MIME attachment handler is assembling a multipart form in order to send a message that includes attachments, the handler generates this string for the new form, and the CreateMimeStream method uses it when assembling the form. 

ProcessMultiPartForm 

CreateMimeStream