Вот заголовки png:
Request URL:https://host.example.com/somedir/some.png
Request Method:GET
Status Code:200 (from cache)
Remote Address:x.x.x.x
Response Headers
accept-ranges:bytes
allow:GET, POST, HEAD
content-length:65811
content-type:image/png
date:Thu, 01 Sep 2016 10:51:31 GMT
etag:"575506e6-10113"
last-modified:Mon, 06 Jun 2016 05:15:18 GMT
server:nginx
status:200

а вот заголовки проблемных файлов:
Request URL:itms-services://?action=download-manifest&url=https://host.example.com/somedir/some.plist
Request Headers
Provisional headers are shown
Upgrade-Insecure-Requests:1
User-Agent:Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/52.0.2743.116 Safari/537.36
Query String Parameters
view source
view URL encoded
action:download-manifest
url:https://host.example.com/somedir/some.plist