liveAgent:clientChatFileTransfer helps Chat visitor to upload documents or attachments when the Agents request it in live Chat Session.

Sample Chat Page:

<apex:page showHeader="false">
    <style>
        body {
            overflow: hidden;
            width: 100%;
            height: 100%;
            padding: 0;
        }
        #liveAgentClientChat.liveAgentStateWaiting #liveAgentQueuePosition {
            display: inline;
        }
        #liveAgentClientChat.liveAgentStateWaiting #waitingMessage {
            display: table;
        }
        #waitingMessage { display: none; }
        #liveAgentQueuePosition { display: none; }
        .liveAgentChatInput {
            height: 25px;
            border-width: 1px;
            border-style: solid;
            border-color: #000;
            padding: 2px 0 2px 4px;
            background: #fff;
            display: block;
            width: 99%;
        }
        .liveAgentSendButton {
            display: block;
            width: 60px;
            height: 31px;
            padding: 0 0 3px;
        }
        #liveAgentChatLog {
            width: auto;
            height: auto;
        }
        .center {
            margin-left: auto;
            margin-right: auto;
            vertical-align: middle;
        }
    </style>
    <div style="margin: 5px;">
        <liveAgent:clientChat >
            <liveAgent:clientChatSaveButton />
            <liveAgent:clientChatEndButton />
            <div>
                <liveAgent:clientChatAlertMessage />
                <liveAgent:clientChatStatusMessage />
                <table cellpadding="10" cellspacing="10" id="waitingMessage">
                    <tr>
                        <td colspan="2">
                            Please wait while you are connected to an available agent.
                        </td>
                    </tr>
                    <tr>
                        <td>Your queue position is </td>
                        <td>
                            <liveAgent:clientChatQueuePosition />
                            <style>
                                span.liveAgentQueuePosition:empty::before { content: '0'; }
                            </style>
                        </td>
                    </tr>  
                </table>
                <div>
                    <liveAgent:clientChatLog />
                </div>
                <div>
                    <liveAgent:clientChatFileTransfer/>
                </div>
                <div>
                    <liveagent:clientChatInput />
                    <liveAgent:clientChatSendButton />
                </div>
            </div>
        </liveAgent:clientChat>
    </div>
</apex:page>

Output: