Upload big file in IIS 8.5 by Uploadify returns 400 ERROR

Hi everybody, I got this problem when I use Uploadify to upload big files(>28.6M), I use Fiddler to watch the Http Response and returns " 400 Your request has bad syntax or is inherently impossible to satisfy."

I have set IIS_Schema.xml "requestLimits->maxAllowedContentLength"  to "300000000", but it still not working.

<element name="requestLimits"><attribute name="maxAllowedContentLength" type="uint" defaultValue="3000000000" /><attribute name="maxUrl" type="uint" defaultValue="4096" /><attribute name="maxQueryString" type="uint" defaultValue="2048" /><element name="headerLimits"><collection addElement="add" clearElement="clear" removeElement="remove" ><attribute name="header" type="string" required="true" isUniqueKey="true" validationType="nonEmptyString" /><attribute name="sizeLimit" type="uint" required="true" /></collection></element></element>

And Web.config

<security><requestFiltering><requestLimits maxAllowedContentLength="300000000" /></requestFiltering></security>

This problem has confused me serveral days , can any body give me some suggestions, Great thanks.