What?
A quick article on making an "upload file" field mandatory.

Why?
Because for some reason, it wouldn't appear as selectable when I tried using a layout rule. Probably everyone else has figured this out and I'm just late to the party.

How?
Through the wonders of Zoho CRM Client Script.

So we're going to check on page onload, onchange for the Create Page (Standard), Edit Page (Standard), and Details Page (Standard).

My use-case has a requirement that another dropdown called "Order Type" has to be set to "Supplier". My file upload field is called "Order Confirmation".

v_OrderType = ZDK.Page.getField('Order_Type').getValue();

if (v_OrderType == "Vendor")
{
    var o_UploadField = ZDK.Page.getField('Order_Confirmation');
    o_UploadField.setMandatory(true);
}