Edit in JSFiddle

//because read.csv is in utils
ocpu.seturl("//public.opencpu.org/ocpu/library/utils/R")

//actual handler
$("#submitbutton").on("click", function(){

    //arguments
    var myheader = $("#header").val() == "true";
    var myfile = $("#csvfile")[0].files[0];
        
    if(!myfile){
        alert("No file selected.");
        return;
    }

    //disable the button during upload
    $("#submitbutton").attr("disabled", "disabled");

    //perform the request
    var req = ocpu.call("read.csv", {
        "file" : myfile,
        "header" : myheader
    }, function(session){
        session.getConsole(function(outtxt){
            $("#output").text(outtxt); 
        });
    });
        
    //if R returns an error, alert the error message
    req.fail(function(){
        alert("Server error: " + req.responseText);
    });
    
    //after request complete, re-enable the button 
    req.always(function(){
        $("#submitbutton").removeAttr("disabled")
    });        
});    

<h1>Uploading a CSV File</h1>

<b>CSV File</b> <input type="file" id="csvfile"> 
<br />

<b>Header</b> <select id="header">
<option>true</option>
<option>false</option>
</select> 

<br />
<button id="submitbutton" type="button">read.csv</button>

<pre><code id="output"></code></pre>