ASP.NET WebApi POST not recognizing form-urlencoded int array

So this post gives an answer to how to send a post for downloading a file using forms. But my API endpoint isn't recognizing the form data.

[HttpPost]
public async Task<IHttpActionResult> DownloadPdfs([FromBody] IEnumerable<int> ids)
{
    if (ids == null || ids.Count() < 1)
    {
        return BadRequest("No ids supplied.");
    }
    ...

'ids' always has a count of 0.

Here's the JS code:

function downloadFile(ids) {    
  var win = 'w' + Math.floor(Math.random() * 1000000000000);
  window.open('', win, 'width=250, height=100');
  var f = $('<form></form>')
      .attr({ target: win, method: 'post', action: 'api/pdfs' })
      .appendTo(document.body);
  _.each(ids, function (id) {
      $('<input></input>')
          .attr({ type: 'hidden', name: 'ids', value: id })
          .appendTo(f);
  });
  f[0].submit();
  f.remove();
}

Initially, I just had it the same as the other linked answer which was just one input appended to the form, in which I set the value to 'ids'. I tried this next, but that still didn't work.

Does anyone know how to adapt this to still use post data to provide my ids, which are needed to create and supply the downloaded file? Or is there a better way to do this?

I've tried doing it just using Ajax, which returns the file content in the responseText, but I can't make use of it in there and I can't get the browser to open it.

Edit 1

• I also just tried setting the value to this:
  value: '{ids: [' + ids.toString() + ']}'
  That didn't work.

Edit 2

• Just tried changing the endpoint parameter to this:
  ([FromBody] int[] ids)
  ...to no avail.