attachment file in send mail task

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ASP.NET

IIS

C#3.5

i have task in send mail and now it work good but when i want to develop this tasks to attach file to send mail i get this error.
any one can help me to erase this error please.
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ASP.NET is not authorized to access the requested resource. Consider granting access rights to the resource to the ASP.NET request identity.
ASP.NET has a base process identity (typically {MACHINE}\ASPNET on IIS 5 or Network Service on IIS 6) that is used if the application is not impersonating.
If the application is impersonating via <identity impersonate="true" />, the identity will be the anonymous user (typically IUSR_MACHINENAME) or the authenticated request user.

To grant ASP.NET access to a file, right-click the file in Explorer, choose "Properties" and select the Security tab. Click "Add" to add the appropriate user or group. Highlight the ASP.NET account, and check the boxes for the desired access
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