What type of file is the download (e.g. zip, pdf)? The reason I ask is that there's a file named /pub/.htaccess that restricts the filetypes for the download. Here's what it looks like for v1.5.0:
#
# @copyright Copyright 2003-2011 Zen Cart Development Team
# @license http://www.zen-cart.com/license/2_0.txt GNU Public License V2.0
# @version $Id: .htaccess 18695 2011-05-04 05:24:19Z drbyte $
#
## the following line is needed to allow Download-By-Redirect to work
Options +FollowSymLinks
#
# This is used with Apache WebServers
#
# The following blocks direct HTTP requests to all filetypes in this directory recursively, except certain approved exceptions
# It also prevents the ability of any scripts to run. No type of script, be it PHP, PERL or whatever, can normally be executed if ExecCGI is disabled.
# Will also prevent people from seeing what is in the dir. and any sub-directories
#
# For this to work, you must include either 'All' or at least: 'Limit' and 'Indexes' parameters to the AllowOverride configuration in your apache/conf/httpd.conf file.
# Additionally, if you want the added protection offered by the OPTIONS directive below, you'll need to add 'Options' to the AllowOverride list, if 'All' is not specified.
# Example:
#<Directory "/usr/local/apache/htdocs">
# AllowOverride Limit Options Indexes
#</Directory>
###############################
# deny *everything*
<FilesMatch ".*">
Order Allow,Deny
Deny from all
</FilesMatch>
# but now allow just *certain* necessary files:
<FilesMatch ".*\.(zip|ZIP|gzip|pdf|PDF|mp3|MP3|swf|SWF|wma|WMA|wmv|WMV|wav|epub)$">
Order Allow,Deny
Allow from all
</FilesMatch>
<IfModule mod_headers.c>
<FilesMatch ".*\.(zip|ZIP|gzip|pdf|PDF|mp3|MP3|swf|SWF|wma|WMA|wmv|WMV|wav|epub)$">
# tell all downloads to automatically be treated as "save as" instead of launching in an application directly
# ALERT: ForceType requires Apache2 or later. If using older version of Apache, it will need mod_mime installed. Or just comment out the ForceType line below
# (to disable, just comment the next 2 lines by adding a '#' at the beginning of each):
ForceType application/octet-stream
Header set Content-Disposition attachment
</FilesMatch>
</IfModule>
IndexIgnore */*
## NOTE: If you want even greater security to prevent hackers from running scripts in this folder, uncomment the following line (if your hosting company will allow you to use OPTIONS):
# OPTIONS -Indexes -ExecCGI
If your download file's filetype is not listed in the statements I marked in red, just add your filetype to the lists (without the .) and reupload the .htaccess file.
