Difference Between Onkeypress And Onkeydown?

I have a form with a default button set.So when i press enter,form is submitted regardless of the focus. My requirement is when focus is inside fileupload control and enter is pressed,instead of submitting the form show browse dialog. So I created a panel and attached a onKeyPress event to this panel

fileUploadDocument.Attributes.Add("onkeyPress", "return ByPassEnter(event);");
<asp:Panel ID="fileUploadDocument" runat="server">
<div>
<uc2:FileUploadControl ID="ucdocxUploadControl" runat="server" ErrorMessageNoFileSelected="select a file for upload."
ValidationGroup="BrandLogoFileUpload" />
</div>
</asp:Panel>
function ByPassEnter(e) {
var evt = (e) ? e : window.event;
var key = (evt.keyCode) ? evt.keyCode : evt.which;
if (key == 13) {
document.getElementById('<%=ucdocxUploadControl.BaseFileUploadControl.ClientID %>').click();
CancelDefault(evt);
}
}
function CancelDefault(e) {
e.cancel = true;
if (e.preventDefault) {
e.preventDefault();
}
e.returnValue = false;
}

When i press enter on fileupload control, browse dialog shows up and when i choose a file or hit cancel on browse dialog,postback happens. e.returnValue = false should prevent the postback but even then postback happens. However if i change onkeypress to onkeydown,everything works and postback doesnot happen.