Chapter 15 Unified Threat Management for FortiOS 5.0 : Web filter : Advanced web filter configurations : Web resume download block
  
Web resume download block
Enable to prevent the resumption of a file download where it was previously interrupted. With this filter enabled, any attempt to restart an aborted download will download the file from the beginning rather than resuming from where it left off.
This prevents the unintentional download of viruses hidden in fragmented files.
Note that some types of files, such as PDF, fragment files to increase download speed and enabling this option can cause download interruptions. Enabling this option may also break certain applications that use the Range Header in the HTTP protocol, such as YUM, a Linux update manager.