I want to automatically open an e-mail with an attached file ready for sending. Opening the e-mail works but the attachment is not present. Any ideas?

Do you really need the "file://"? What happens if you replace spaces with %20?

Good suggestions. I've tried without "file://". Doesn't work. Another try without any spaces also doesn't work. I wonder if the attachment thing is a limitation of Modest or whatever. Other applications can send e-mails with attachments, like the built-in PDF viewer or 'Maemo Recorder' from garage.maemo.org.

Modest does support attachments but that dosen't mean the mailto protocol implemented in modest does. You are at the liberty of the email client to implement the mailto protocol and include parsing for the attachment clause. You may not know what mail client is installed on the PC, so it may not always work - Outlook certainly doesn't support attachments using mailto.