I have custom dashboard which uses AngularJS. Here is the code I use to post file to ApiController:

angular.module("umbraco").controller('myDriveDashboardController',
function myDriveDashboardController($scope, $http) {

    $scope.uploadFile = function () {
        var fd = new FormData();
        fd.append('file', $scope.myFile);

        $http.post('/umbraco/api/AlarmsImportApi/ImportAlarms?culture=en-US', fd, {
            withCredentials: true,
            headers: {
                'Content-Type': undefined
            },
            transformRequest: angular.identity
        }).then(function (response) {
            console.log(response);
        }, function (error) {
            console.log(error);
        });
    };
});

When this request reaches API controller method ImportAlarms the file is missing. It seems that FormData is not being correctly serialized to binary because in Chrome tools I can see that request content-type: is set to text/plain;charset=UTF-8. But as I understand it should be 'multipart/form-data'. Setting content-type manually to 'multipart/form-data' does not help. Here is my ApiController:

public sealed class AlarmsImportApiController : UmbracoApiController
{
    public AlarmsImportApiController() { }

    [HttpPost]
    public IHttpActionResult ImportAlarms(String culture)
    {
        HttpRequestBase httpRequest = UmbracoContext.HttpContext.Request;
        if (httpRequest.Files.Count == 0)
        {
            return StatusCode(HttpStatusCode.BadRequest);
        }
        return StatusCode(HttpStatusCode.OK);
    }
}

As you may guess I'm always hitting BadRequest as Files.Count == 0.

This behavior is reproducible with brand new Umbraco 8.6.4 site. I have simply added custom dashboard and ApiController.

Could you please share some thoughts on where I made a mistake?

P.S. Created repository with my project for better understanding: https://github.com/omoravskyi/UmbracoPostFileIssue