When uploading large files into SharePoint document library an error occurs even though file size is set to 50MB in SharePoint Central Administration.This is a known issue. To fix the problem do the following (or rather have your system administrator do it):

1. Open your Web applications web.config file (typically found at C:inetpubwwwrootwssVirtualDirectories80);
2. before closing tag add/modify the following code to set maximum file size to 50MB: 

 <system.net>
  <defaultProxy>
   <proxy autoDetect="true" />
  </defaultProxy>
 </system.net>
 <system.webServer>
  <security>
   <requestFiltering>
    <requestLimits maxAllowedContentLength="52428800"/>
   </requestFiltering>
  </security>
 </system.webServer>
</configuration>

maxAllowedContentLength sets a maximum file size in bytes.