Application does not appear in Outlook 2016

Question

I created an email with Windows.ApplicationModel.Email.EmailMessage, and I added an application with the code below:

var stream = RandomAccessStreamReference.CreateFromFile(file);
var attachment = new EmailAttachment(file.Name, stream);
emailMessage.Attachments.Add(attachment);
await EmailManager.ShowComposeNewEmailAsync(emailMessage);

In the default email client found in Windows 10, the attachment appears as an attachment. However, the application does not appear at all in Outlook 2016 with the same code. Any suggestions how to fix this?

EDIT: So, this seems to be a problem with Windows 10 and all email clients except the standard Windows 10 email client, according to this .