image upload through json in rails3 - ruby-on-rails

Question

I'm using carrier wave for image upload and trying to do upload
image through json,but its not going to save value into database
.While image name is coming in controller correct here
#Employee_events.external_doc=params[:external_doc] .But its not
going to set in this #Employee_events.external_doc variable , as i
thing may be issue will be multipart. But d'not know how to use in
json.
.js file
$(document).ready(function() {
var evnt = {};
$("#Event_dialog-form").dialog({
autoOpen : false,
height : 400,
width : 600,
modal : true,
buttons : {
"Enter" : function() {
evnt.external_doc = $('#imgid').val();
$.post("/employee_management/add_event", evnt, function(data) {
if (data) {
alert("Data entered successfully");
} else {
alert("Oops! some error occured");
}
});
return false;
},
Cancel : function() {
$(this).dialog("close");
}
},
close : function() {
//allFields.val( "" ).removeClass( "ui-state-error" );
}
});
$(".EventEntryDialog").click(function() {
var Ed = $(this).attr("id");
$('#user_id').val(Ed);
$("#Event_dialog-form").dialog("open");
});
});
Controller
def add_event
result=TRUE
#Employee_events=EmployeeEvent.new
#Employee_events.external_doc= params[:external_doc]
puts #Employee_events.external_doc.to_s
respond_to do |format|
if #Employee_events.save
format.json { render :json => result }
else
return FALSE
end
end
end