Thanks for your answer.
I have the page "Files.htm" in pages/dropdown/ of "extranet" folder but not in my custom. By the way I don't have any htm files in my custom, no need.
I assume that if the issue is linked to rights access, I should have the error page saying that I have no right to see the element or something like this ?
When I click on the attachment picture, nothing happens ; there is no request in the network tab of Chrome developer console.