Attachments in "mailto:" protocol: In an Image-command I have added a start= parameter that opens up an email client with the "mailto:" protocol. This works fine, but I don't succeed in adding an attachment automatically with &attachment=... What could be the reason?

The "&attachment=" attribute is not part of the official "mailto:" protocol specification.  Some email clients used to support attachments in the past, e.g. MS Outlook, but newer versions of MS Outlook and other email clients no longer do so, for security reasons. See e.g. http://stackoverflow.com/questions/21147296/does-outlook-2011-allow-attachments-through-mailto-link. Perhaps you can implement an alternative using our "Control" component as described in our  Tips&Tricks Nr. 44. It is based on VBScript or VB.NET and allows attachments.