AddMailAttachmentData()

[jassing]

The type of the attached file. If this parameter is omitted, then the file extension will be used to determine that files mime type.

You're attaching data - there is no file name, and hence, no file extension.

[Andre]

You're right about that there is no file.

But how the description of this optional parameter 'MimeType$' should look like?

[jassing]

I would think it should not be optional when using data.

[Andre]

So this sounds like a feature request, where Fred should take a look...