Foritgate 100D | While scan xlsx, IPS engine causes high CPU and slow download

hi,

we have a Fortigate 100D with FortiOS 5.2.6. The problem is, when we download a xlsx file from our server via HTTP/S, the download starts fast, then it goes down to ~30KB/s. We can see, that the CPU gets high, with the IPS engine process.

Now we try to find out, why and how we could disable the scan/IDS from xlsx/zip files. Hopefully, someone can push us in the right direction.