Feature request: Make multiple file upload to custom URLs (eg S3 presigned URLs) simpler

[mariusa]

Hi,

Googling various resources, I finally got working multiple file upload to S3 presigned URLs (using Vue)
This involves:

1. custom accept handler, which gets S3 presigned URL for each file and sets it as a file property, eg file.uploadUrl

2. manual processing: autoProcessQueue: false

3. on('processing', ) handler

let onDropzoneProcessing = (file) => {
    cl('onDropzoneProcessing', file.uploadUrl)
    // change url before sending
    //this.options.url = file.uploadUrl this doesn't work, must use global variable as below
    myDropzone.options.url = file.uploadUrl //so only 1 at a time?
}

4. overwrite sending (can't tell why, multiple answers says it's required)

sending: function (file, xhr) {
                let _send = xhr.send
                //console.log('sending', file.name, file.uploadUrl)
                //this.options.url = file.uploadUrl no effect, still uploading to #
                xhr.send = function () {
                    _send.call(xhr, file)
                }
            },

Docs https://docs.dropzone.dev/configuration/basics/configuration-options
mention for url that You can also provide a function that will be called with files and must return the url

Ideally, this should all that be needed. But it seems url is the same for all files, so it doesn't work with multiple uploads.
It also needs to support async, like accept does, by having a callback parameter.

Next ideal solution would be just step 1 above.
Then, if a file has a special property set, eg uploadUrl, have dropzone use that to upload the file to. That's it.
It does seem simple to implement and would save a lot of headaches for new users with this use case.

What do you think?