Two things that you need to consider while sending an email with attachment.
1. The file that is going to be attached has to have read permission. You already discussed with Naerling.
2. The size of the file has to checked against the actual size permitted to process by IIS worker process. This can be set from the
file of your web application.
<httpRuntime maxRequestLength="102400" executionTimeout="5400"/>
Hope this will help you well.