Corrupt downloads - html appended

On some products (e.g. multi-attribute), the downloaded .exe files are being corrupted with additional html content being added. Other .exe downloads are fine. This WAS working, but was being administered by a contractor and his gurus who have now deserted the scene. So now we are picking up the pieces.
Downloading the file via ftp works fine. Have reproduced with single attribute product. Not afraid to get into the databases to provide diagnostics.

Using ZC 1.3.6 with "Download by Redirect" off, as otherwise, top and bottom of file have extra html.

All help appreciated.
Cheers from NZ

Please describe the "extra html" that is on the files.
Have you tried enabling Download by streaming ?

Thanks for the response. Download by streaming set to true doesn't work either. Some of the downloads that work are 12Mb, which is 5MB larger than those that are failing.
HTML is attached txt file also.

Cheers,

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" dir="ltr" lang="en"> <head> <title>Your Download ...</title> <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" /> <meta name="keywords" content="Software Gift Certificates Your Download ..." /> <meta name="description" content="Your Download ... - Software Gift Certificates " /> <meta http-equiv="imagetoolbar" content="no" /> <meta name="author" content="Sprite Mobile Ltd" /> <meta name="robots" content="noindex, nofollow" /> <base href="http://www.spritesoftware.com/store/" /> <link rel="stylesheet" type="text/css" href="includes/templates/Sprite_template/css/stylesheet.css" /> <link rel="stylesheet" type="text/css" href="includes/templates/Sprite_template/css/stylesheet_css_buttons.css" /> <link rel="stylesheet" type="text/css" href="includes/templates/Sprite_template/css/

You said you're using Zen Cart v1.3.6.
What addons/contributions have been installed?
What kinds of customizations have been done to the site?

I'd be inclined to do a complete comparison of your site's files vs the master/original files to see what might be causing this. Something akin to this procedure:
http://www.zen-cart.com/wiki/index.php/Troubleshoot_-_Diagnosing_Obscure_Issues

Might be easier if someone could tell me where the code for the download button usually sits and we'll start debugging. I don't know all the customizations that happened - external contractors set up the cart for us and comparing source code seems like a blunt instrument given that we wouldn't recognize what changes were previously made as part af the customization, and what changes are related to the download problem scenario. Given that it works fine on some product downloads and not others, I would have thought that would help identify the cause of the problem....
How about low memory conditions ?
Thanks,

download-by-streaming handles the low-memory situation.

Compare this file as a start:
/includes/modules/pages/download/header_php.php

However, that file might be perfectly fine, but the problem stems from somewhere else.
Technically if that file is operating properly, then all the template-related content that's getting into your files should never get there, because the template never triggers if this file's code is running normally.

Do you have some sort of SEFU/SEO Url module installed that could be interrupting the normal load process?

Thanks. Will see what I can find today.