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Office 365 CDN has content-disposition=attachment for JS files

Martin Laplante 316 Reputation points
2020-08-28T20:59:53.513+00:00

I am trying to resolve a problem where someone's browser is sometimes downloading a js file rather than executing it. I thought I would compare their http headers with mine, and I see that even the headers on mine don't seem to be correct.

On a file created by an SPFx extension, whose code is stored in ClientSideAssets and served by publiccdn.sharepointonline.com, I see the response header 

content-type: application/javascript

which is normal, but also 

content-disposition: attachment; filename={LongFileName}.js

which seems wrong to me. Most browsers use the content-type to determine whether to process the file or download it, and in my browser everything is fine since the content-type isn't "Application/octet-stream", but my understanding is that the standard says that "content-disposition: attachment" means "download this file". Maybe some browsers follow the standard.

Can someone who knows more about this tell me whether the O365 CDN is configured incorrectly?

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  1. Martin Laplante 316 Reputation points
    2020-09-09T03:16:59.847+00:00

    I have tried it without a CDN. It also returns content-disposition: attachment; filename={LongFileName}.js for certain files, notably the "...strings_en-US_....js"
    files, but not for other JS files. Very strange.

    We have one user who is occasionally prompted to download that JavaScript file.

    This seems to happen for some SPFx app JS files but not others. Very strange.