Resource overrideMimeType not working on Edge

TL;DR: I found a spec-compliance issue with how Resource does XHRs.

I came here to report an issue I was having with Edge, but to my surprise it isn’t Edge’s fault. I have some code that makes a Resource and calls fetchXML, treating the response as an XMLDocument. This failed in Edge, which led me to dig a bit – I noticed that my server was returning the (definitely valid XML, not HTML) content with a Content-Type of text/html. I don’t know why, it’s not under my control, so I’m stuck with it. Firefox and Chrome return an XMLDocument anyway; Edge was returning an HTMLDocument. I thought this was the bug, until I did a bunch of extra testing.

I found out that the XHR spec was recently updated to state that calling open() must reset the “override MIME type” – see bullet 12 in the current spec. I saw that the Resource class calls overrideMimeType before calling open. This results in the override getting ignored in recent builds of Edge, and the problem will get worse as other browsers come into compliance with the changes.

I believe the fix is as simple as moving the open call before the call to overrideMimeType. This would be a PR instead of an issue, but my CLA issues remain unresolved, so I’ll just point 👉

[hpinkos]

Thanks again for reporting this @thw0rted! We just merged a fix for this into master and it will be included in the 1.50 release on Monday

[mramato]

Thanks @thw0rted! I was able to replicate this behavior and opened #7091 with the fix.