= XMPP file-upload not working.

== Problem

When I want to upload a photo to a group-chat using Gajim, I can see on the server that a directory is created to receive the image, but the image never gets there.
Thus, Gajim reports an empty file (actually a 404 error, according to Nginx).

== Configuration

My PC (XMPP client) runs Gajim on Archlinux.
The PC has IP 192.168.1.99.

The server runs Archlinux too.
Here is the network setup:

[ditaa]
-------
        +-----------------------------------------------------------------+
        | Server                                                          |
        |  +-----------+  +----------------+  +-------------------------+ |
+----+  |  | nftables  |  | haproxy (tcp)  |  |  nginx : /…/https+.pp   | |
| PC +->+->+ :443 dnat +->+ :444 tls_plus  +->+   ↓                     | |
+--+-+  |  | → :444 OK |  | → /…/https+.pp |  | /---------------------\ | |
   |    |  +-----------+  +----------------+  | |       ssowat        | | |
   |    |                                     | | "/x…": pass-through | | |
   |    |  +---------------+                  | \---------------------/ | |
   |    |  |    prosody    |                  |   ↓                     | |
   \--->+->+ :5222   :5280 +<-----------------+  location /xmpp- { … }  | |
        |  +---------------+                  +-------------------------+ |
        |                                                                 |
        +-----------------------------------------------------------------+
-------

== Versions

PC::
    * Archlinux kernel 4.15.11
    * Gajim 1.0.0
    * Gajim plugin httpupload 0.4.6

Server::
    * Archlinux kernel 4.15.11
    * nftables 0.8.3
    * haproxy 1.8.4
    * nginx 1.13.10
    * ssowat-git (my fork)
    * prosody 0.10.0
    * prosody-mod-auth-external-hg r2944.37ec4c2f319a
    * prosody-mod-auto-accept-subscriptions-hg r2944.37ec4c2f319a
    * prosody-mod-csi-hg r2944.37ec4c2f319a
    * prosody-mod-filter-chatstates-hg r2944.37ec4c2f319a
    * prosody-mod-http-upload r2944.37ec4c2f319a
    * prosody-mod-mam-adhoc 0.10.0
    * prosody-mod-offline-email-hg r2944.37ec4c2f319a
    * prosody-mod-smacks 2017.08.27
    * prosody-mod-throttle_presence r2944.37ec4c2f319a