ErikMattheis

Is it thumbnails to media uploads that are broken or other images in the admin like icons that come with WP?

If the latter, I had a similar issue that occurred when viewing the admin over HTTPS – it would loads assets over HTTP, causing them to not be displayed in the browser. Installing the plugin SSL Insecure Content Fixer fixed the issue.

I’m still stuck on this. Anyone have any ideas at all?

Does anyone need any more information to come up with a possible explanation?

Thanks.

The content-type was being sent by a file in the theme. Unsure why it only happens with these W3TC requests to admin.php.