How to Pass an Attachment to a method in webservice

[PulkitZery]

Hi,
I am trying to set up a mailing system for my company, where uses send an email from their desktop client applications and that calls a method defined in web service to send the email out.

Currently I am stuck on a problem, about how to send attachments to the method I defined in the web service? If I define MailAttachment as a parameter in my method, it gives me the following error:
System.Web.Mail.MailAttachment cannot be serialized because it does not have a default public constructor.

Moreover, attachments could be one or they could be many, I have to deal with this issue as well. Also usually in our company users send big attachments like 10MB – 20MB.

Please help me out, I have to develop this ASAP. Thanks very much in advance for your help.

Jerry