[upload] IE11 uploading multipart form data need to be in a specific order

[sey]

Hi,

When uploading a file on IE11 (not yet tested on other IE or edge versions) the different parts of the upload need to be in a specific order.
Meaning if you upload a file and add some other information like an ID, the file need to be last in the form data. Nothing is sent after the file.
This looks very much like the known issue of pristine checkboxes being last in a form which are not sent with IE.

I posted the issue on stackoverflow recently.

The quickfix for this is to allow appending data to the FormData object before the file is added to the FormData. Passing the formData object to the upload-before event did the trick for me.

const evt = this.dispatchEvent(
    new CustomEvent('upload-before', {
        detail: {file, xhr, formData},
        cancelable: true
    })
);

Before submitting a pull request, I'd like to know if that fix is ok with you. Or if there are specific reasons why the formData was not passed originally for that event.

[platosha]

Hi, sorry for late response and thanks for informing us. We were not aware of such an issue with FormData order in IE.

We don’t have any specific reasons to not pass a FormData in upload-before.

I couldn’t reproduce a same issue with my IE11 (Windows 10). Receiving the parts after the file worked for me.

I understand that for some use cases the order of the FormData parts might be a concern anyway. So it seems to be a good idea to pass a formData in upload-before to allow for prepending data parts before the file.

[rwestlund]

I need this for my application too. My metadata part must come before the file part in order to determine what to do with the incoming file stream. @sey's fix looks like the right way to do this.