Ejabberd XMPP server includes a SOCKS5 proxy, setting it up correctly is what makes the difference between fast or very slow file transfer operations.
As per XEP-0065 file transfer is either peer-to-peer or mediated by a proxy server.
In Conversation peer-to-peer transfer is done by converting the file in base64, split it in 4 kb chunks sent one at the time always awaiting first for the ACK of the precedent one; this makes the whole process painfully slow and bandwidth consuming.
The other file transfer method supported by Conversation is defined by XEP-0234 (or Jingle file transfer) which relies on a SOCKS5 proxy and also allow to negotiate parameters like encryption.
Ejabberd configuration:

acl:
  proxy_users:
    server:
      - "hostname.domain"
	
access_rules: 
  proxy65_access:
    - allow: proxy_users
  proxy65_shaper:
    - none: admin
    - proxyrate: proxy_users
	
shaper: 
  normal: 100000
  proxyrate: 1000000
	
modules: 
  ...
  mod_proxy65:
    host: "proxy.hostname.domain"
    name: "File transfer proxy"
    ip: "127.0.0.1"
    port: 7777
    max_connections: 5
    access: proxy65_access
    shaper: proxy65_shaper
  ...

Instruct iptables to allow traffic on SOCKS5 proxy port:

INPUT -p tcp -m tcp --dport 7777 -j ACCEPT

Restart Ejabberd and reboot the phone or manually terminate (Settings -> Apps) and reopen Conversations.