File uploads are broken on latest Firefox Nightly

[k80w]

Under the latest version of Firefox Nightly, upon uploading a file, the following text is always uploaded in place of the files real contents

function stream() {
    [native code]
}

This seems to be caused by http-api.js:70:

const body = file.stream ? file.stream : file;

On other browsers, File.stream will be undefined; however, Firefox Nightly now adheres to the latest draft of the File API which defines a .stream() method, meaning this line is swapping the content of the file for this stream method and later stringifying and uploading that method.

The original purpose of this line seems to be to keep compatibility with older code, which was supposedly encouraged to set file.stream to the stream to be uploaded. I'm not sure if this is still necessary, especially considering this is apparently undocumented behavior. Deleting this line entirely solves the issue.

If it is necessary to keep compatibility, we could just ensure the file isn't a File before checking for file.stream.

This may be the same issue as #908, as the description of that issue sounds similar.

[turt2live]

Highly related (by error): element-hq/element-web#9924

Going to raise this as a fire because it looks like we aren't handling files correctly at all.