To allow large attachments to be uploaded add change of override the MaxRequestLength parameter in Machine.config or Web.config.  For instance I added the following value to Web.config to allow uploads of up to 8MB:

<httpRuntime maxRequestLength="8192" />

So the full line read:

<httpRuntime useFullyQualifiedRedirectUrl="true" maxRequestLength="8192" />

See http://support.microsoft.com/default.aspx?scid=kb;en-us;Q295626