We have developed our own attachments handler for use in Fiori apps. The upload process is working everywhere and the download works in Chrome for all files and internet explorer for everything except office document types (docx, xlsx etc).
When you go to a URL for a word document in IE, it asks if you want to open it or download it. If you choose download, all is well, if you choose open, IE passes the request off to the relevant office application. When this happens, "Microsoft Office Protocol Discovery" kicks in, and office sends an OPTIONS request to the gateway server. This request causes an authentication window to popup.
If the user cancels the popup, the downloaded attachment opens fine, but I'd really like to remove the popup. Has anyone encountered this before?
Details on what office is doing can be found here: