The file transfer feature is not currently implemented in CAXL.
However, per the Cisco IM&P documentation, it will route requests containing the XEP-0096 'file transfer' XMPP extensions (i.e. not block/reject), however Cisco IM&P does not participate at all in the actual interpretation or processing of XEP-0096. If both endpoints support XEP-0096, they could send low-level XMPP messages (via CAXL) to each other which contain the XEP-0096 schema elements, and therefore negotiate and implement file transfer between themselves.
There is no higher-level function in CAXL exposed for file transfer, the apps would need to build and send lower-level XMPP stanzas with XEP-0096 contents.
A Cisco user added code to jabberwerx.js (CAXL) to support file transfer. This is not from Cisco and is not supported by Cisco, but you may find it useful. See attachment.