If you see the rather unhelpful HTTP Error when uploading large files of any type to an IIS based WordPress using FastGCI and have tried all the things on the web about tweaking PHP.ini settings without success, try the following:
- Open Internet Information Services (IIS) Manager
- In the Connections pane, select the server
- In the Home page, double-click Request Filtering
- Click Edit Feature Settings… in the Actions pane.
- Change the Maximum allowed content length (Bytes) from 30000000 (30MB) to a desired value 300000000 (300MB)
- Click OK
The change will action straight away, no need to restart anything.
Try your upload, success!
Make sure you try Fixing errors when uploading images in WordPress first, it might be a simple PHP issue.