When using IIS with WordPress I kept getting errors with uploading files larger then about 30MB with a “HTTP error” in the uploader. Troubleshooting this problem on the internet led me on a wild goose-chase for settings in WordPress or PHP which already all where good. Finally after turning on detailed error messages and logging through Fiddler I found that the culprit was actually the IIS Request Filtering module.
Going into Request Filtering > Edit Feature Settings… you find a setting “Maximum allowed content lenght (Bytes):” which is by default on nearly 30MB in bytes. I changed this to my desired setting that maxed PHPs upload_max_filesize and post_max_size. All works fine now!