ajax loaded content not cached

I tweaked boost to cache the different content types I need: jpeg, png, gif, pdf and swf.
This works when the files are directly linked in pages.
However, it fails when trigged by ajax. The header only see the JSON code, not the called content.
See for example this page https://www.see.asso.fr/fichier/18857_energie-nucleaire-solution-avenir-....
The swf preview and other image format are not catched.
below the headers for the swf preview:

https://www.see.asso.fr/file_preview/file_video_flash_swf/b4413a3190fd4255afc376c717783717a1756a09/31896
POST /file_preview/file_video_flash_swf/b4413a3190fd4255afc376c717783717a1756a09/31896 HTTP/1.1
Host: www.see.asso.fr
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:52.0) Gecko/20100101 Firefox/52.0
Accept: application/json, text/javascript, */*
Accept-Language: fr,en;q=0.8,fr-FR;q=0.5,en-US;q=0.3
Accept-Encoding: gzip, deflate, br
X-Requested-With: XMLHttpRequest
Referer: https://www.see.asso.fr/fichier/18857_energie-nucleaire-solution-avenir-durable-bas-carbone
Content-Length: 0
Content-Type: text/plain;charset=UTF-8
Cookie: _ga=GA1.3.2035831487.1489742437; has_js=1; SESS49d6a4b14bb30beb24db03b809981c0d=c5cc760eb5602abec50f93cc25628e2b; _gat=1
Connection: keep-alive

HTTP/1.1 200 OK
Date: Wed, 22 Mar 2017 13:17:59 GMT
Server: Apache/2.2.22
Strict-Transport-Security: max-age=15768000
X-Powered-By: PHP/5.3.10-1ubuntu3.26
Last-Modified: Wed, 22 Mar 2017 13:17:59 +0000
Cache-Control: no-cache, must-revalidate, post-check=0, pre-check=0
Etag: "1490188679"
Content-Location: https://www.see.asso.fr/file_preview/file_video_flash_swf/b4413a3190fd4255afc376c717783717a1756a09/31896
Content-Length: 1057
Keep-Alive: timeout=5, max=500
Connection: Keep-Alive
Content-Type: application/json

Thanks for help