IE adding content-type multipart/form-data to GET requests

问题

I have a form like this:

<form method="POST" enctype="multipart/form-data">
<input type='submit' name='foo' />
</form>

When that form is submitted, I have some PHP that renders an image:

<img src="http://s3-external-1.amazonaws/bucket/example.jpg?AWSAccessKeyId=foo&Signature=bar&expires=111111" />

The problem is that Internet Explorer 11 (and other versions I believe) are sending a Content-Type: multipart/form-data; boundary=---------------------------7dfcd3b0f0a on that GET request.

When AWS receives the request, they say the signature doesn't match (rightly so).

Few weird quirks:

• If I load the page with HTTPS, the content type is not set to any value (so it works)
• This doesn't happen in any other browsers
• If I have the dev tools open in IE, the content type is not set to any value (so it works)
• If I remove multipart/form-data the content type is not set to any value (so it works)

I have tried issuing a cURL request with/without the content-type. It fails with, and succeeds without.

Why would IE be adding a content-type to all GETs on the resulting page after a form submit?

update: It doesn't appear to happen with all forms - but when we submit from a certain part of our application, this happens. Near as I can find, the form on the submitting page is identical, but the form data is put through a different server-side function before the image is rendered. Can't see how this affects IE adding a content-type however.

来源：https://stackoverflow.com/questions/30584155/ie-adding-content-type-multipart-form-data-to-get-requests