Url "documentdownload" does not works in my <iframe

Hello

I have a process with an overview. In the overview, I download a document via an HTTP link
"/bonita/API/"+ contextbdm.grandparent_ref.url

On the portal, I can access a case. From the case, I access the overview; then, I upload the document. Url is
http://localhost:9080/bonita/API/documentDownload?fileName=aDoc%201.pdf&...
HTTP code= 200 / I received the content.

Via the Explorer page (https://community.bonitasoft.com/project/page-explorer-seach-cases-multi...), I do the same manipulation.
I access the Overview on the same case, and I click on the same URL and...
URL to upload is
http://localhost:9080/bonita/API/documentDownload?fileName=aDoc%201.pdf&...
HTTP code is 200, BUT there is not document uploaded. I have the feeling Bonita Portal says "200" but sends an empty document.
HAR is here:
https://www.dropbox.com/s/re7sw27ywfjhph2/CaseOverviewActifBingHtml.har?...

So, I suspected a CORS or anything like this.
I analyze the portal structure: Portal do a

when explorer does:

So, seem the be the same isn't ? Difference is the parent of the Iframe : for the Bonita Portal, it's /bonita/portal, for the custom page, /bonita/portal/custom-page/custompage_explorer/?

Last but not least, if I copy the documentDownload directly from the browser, it's working.
So, any idea on what's its blocking? Is the Bonita portal who send an empty document?