Okay, so "until now" appears to mean "until I tried this new huge file size that I've never used before".
The more information you provide, the easier it is to help ... ie:
What's the file type? (extension)
What's the file size/type for all your other working downloads?
What's your download-delivery method? ie: is Download by Redirect on or off? Is streaming on or off?
If you point it to a smaller sized file, what's the max size that it will serve properly for you?
What version of Zen Cart are you using?
What version of PHP?
What webserver engine? apache/IIS/etc?
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