Ran into this today and saw no one had posted a fix. Don't go editing the functions files.
FILE: includes/templates/YOUR_TEMPLATE/templates/tpl_product_info_display.php
Locate:
echo zen_draw_form('cart_quantity', zen_href_link(zen_get_info_page($_GET['products_id']), zen_get_all_get_params(array('action')) . 'action=add_product'), 'post', 'enctype="multipart/form-data"') . "\n";
Change to (changes in red):
echo zen_draw_form('cart_quantity', zen_href_link(zen_get_info_page($_GET['products_id']), zen_get_all_get_params(array('action','number_of_uploads')) . 'action=add_product'), 'post', 'enctype="multipart/form-data"') . "\n";
The change is to add the "number_of_uploads" parameter to the array of parameters excluded when building a URL with the zen_get_all_get_params function. This makes use of the built-in Zen-Cart function that omits parameters when building a URL (in this case, the URL the visitor ends up at after submitting the "add to cart" form), which should keep anything from breaking.
Note that this will disable all file uploads - so only make the change if you don't ever plan on using the uploads feature on any products you sell.
It may also make the 'enctype='"multipart/form-data"' unnecessary, but doesn't seem to affect my site so I've left it in.