In Prosody 0.12 and later, consider switching to mod_http_file_share which is distributed with Prosody. You can migrate existing files using mod_migrate_http_upload.
With the plugin installer in Prosody 0.12 you can use:
sudo prosodyctl install --server=https://modules.prosody.im/rocks/ mod_http_upload
For earlier versions see the documentation for installing 3rd party modules