Internet Explorer, HTTPS and file downloads: “The file could not be written to the cache”

Published August 17th, 2011

[Please see update below for a proper fix]

Today I received an email from a client that had just moved their site over to SSL (i.e. HTTPS) — they were no longer able to download files in Internet Explorer. When they tried to do so, a message would appear:

Unable to download...[file name]

The file could not be written to the cache

(an alternative error message is “Internet Explorer cannot download [file name]¬†from [site].
Internet Explorer was not able to open this Internet site. The requested site is either unavailable or cannot be found. Please try again later.”)

I tried various suggestions: different response headers, settings to do with saving encrypted files, and even tried (and failed) to install a couple of hotfixes.

In the end, I’d nearly given up when I found this knowledgebase article and decided to have a go at manually adding the BypassSSLNoCacheCheck registry key.

Short story, it worked. It’s not great since it requires the user to do something — I couldn’t fix it server-side — but it was OK in this case because the site was an intranet so their IT people could sort it. Anyway, I hope this helps someone else out there…

UPDATE: OK, after a lot more experimentation I found a proper solution that doesn’t involve registry updates!

It’s a long story but it turns out that PHP was automatically sending an extra header (“Pragma: no-cache”) that I hadn’t noticed. So, I set the headers as follows:

header("Cache-Control: private");
header( "Pragma: private" );

…which overrides the automated headers and appears to solve the problem. Hurray!

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