How can I attach a file to this mailto string?
string mailto = "mailto:" + to + "&SUBJECT=" + subject + "?BODY=" + body +
"&Attachment=" + attachment;
This doesn't work; the file isn't attached.
Remove the quotes at the end of "attachment".
"mailto:" + to + "&SUBJECT=" + subject + "?BODY=" + body + "&Attachment=" + attachment
Where attachment has the attachment link.
Note: This will not work if the users dont have access to the attachment so you can try attaching and sending it through a c# code.