Web API Multi-file Upload Using Dropzone.js Specifying Dynamic Query String

demo.html

<div class="jumbotron">
	<form action="~/api/Upload" class="dropzone" enctype="multipart/form-data" id="dropzoneForm" method="post" name="dropzoneForm">
		<div class="fallback">
			<input multiple name="file" type="file"> <input type="submit" value="Upload">
		</div>
	</form>
</div>

<style type="text/css">
    .dz-max-files-reached {
    background-color: red;
}
</style>

<script type = "text/javascript" >
	Dropzone.options.dropzoneForm = {
		maxFiles: 20,
		maxFilesize: 500,
		init: function() {
			this.on("processing", function(file) {
				// Customize the upload URL by adding querystring
				var baseUrl = $("#dropzoneForm").attr("action");
				this.options.url = baseUrl + "?someParameter=someValue";
			});
			this.on("maxfilesexceeded", function(data) {
				var res = eval('(' + data.xhr.responseText + ')');
			});
			this.on("addedfile", function(file) {
				// Create the remove button
				var removeButton = Dropzone.createElement("<button>Remove file<\/button>");
				// Capture the Dropzone instance as closure.
				var _this = this;
				// Listen to the click event
				removeButton.addEventListener("click", function(e) {
					// Make sure the button click doesn't submit the form:
					e.preventDefault();
					e.stopPropagation();
					// Remove the file preview.
					_this.removeFile(file);
					// If you want to the delete the file on the server as well,
					// you can do the AJAX request here.
				});
				// Add the button to the file preview element.
				file.previewElement.appendChild(removeButton);
			});
		}
	}; 
</script>

UploadController.cs

public class UploadController : ApiControllerBase
{
    public async Task<IHttpActionResult> Post()
    {
        if (!Request.Content.IsMimeMultipartContent())
            throw new HttpResponseException(HttpStatusCode.UnsupportedMediaType);

        try
        {
            var provider = new MultipartMemoryStreamProvider();
            await Request.Content.ReadAsMultipartAsync(provider);

            var baseUploadPath = @"D:\Uploads\" + Request.QueryString["someParameter"];

            foreach (var file in provider.Contents)
            {
                if (file.Headers.ContentLength > 0)
                {
                    var fileName = file.Headers.ContentDisposition.FileName.Trim('\"');
                    var filePath = Path.Combine(baseUploadPath, fileName);
                    var buffer = await file.ReadAsByteArrayAsync();
                    File.WriteAllBytes(filePath, buffer);
                }
            }

            return Ok();
        }
        catch (Exception ex)
        {
            var response = Request.CreateErrorResponse(HttpStatusCode.InternalServerError, ex);
            throw new HttpResponseException(response);
        }
    }
}

This is the code accompanying article at:

https://www.softwarerockstar.com/web-api-file-upload-using-dropzone-with-querystring/