Dear xuguochun,
Please check your web.config file,
there is a config to set HttpRuntime request length limit under <sysmte.web> node
<system.web>
<httpRuntime maxRequestLength="2000000"/>
</system.web>
The default value of maxRequestLength is 4096 KB (4 MB)
you can set a value greater than 70M (71680)
Keep me post
Regards,
Jeff