Custom JS Code to handle Coral multifield for fileupload Resource Type.

.context.xml

<?xml version="1.0" encoding="UTF-8"?>
<jcr:root xmlns:cq="http://www.day.com/jcr/cq/1.0" xmlns:jcr="http://www.jcp.org/jcr/1.0"
    jcr:primaryType="cq:ClientLibraryFolder"
    allowProxy="{Boolean}true"
    categories="[cq.authoring.dialog.all]"/>

multifieldfile-upload.js

(function () {
  var THUMBNAIL_SELECTOR = "[data-cq-fileupload-thumbnail-img]",
    WEB_RENDITION_PREFIX = "cq5dam.web",
    RENDITIONS_PATH = "/jcr:content/renditions",
    IS_FILLED_CLASS = "is-filled";
  var registry = $(window).adaptTo("foundation-registry");
  registry.register("foundation.adapters", {
    type: "foundation-field",
    selector: "coral-fileupload",
    adapter: function (el) {
      var $el = $(el);

      return {
        getName: function () {
          return el.name;
        },
        setName: function (name) {
          if (el.hasAttribute("data-acg-file-upload-ready")) {
            return;
          }
          $el.attr("data-acg-file-upload-ready", true)
          el.name = name;
          itemNodePath = name.match(/(.*)\.\/.*/)[1];
          // update input names
          $el.find("input").each(function () {
            $input = $(this);
            var inputName = $input.attr("name");
            if (inputName) {
              $input.attr("name", itemNodePath + inputName)
            }
          });
          // fix thumbnail
          _fixFileUploadThumbnail($el)
        },
        isDisabled: function () {
          return el.disabled;
        },
        setDisabled: function (disabled) {
          el.disabled = disabled;
        },
        isInvalid: function () {
          return el.invalid;
        },
        setInvalid: function (invalid) {
          el.invalid = invalid;
        },
        isRequired: function () {
          return el.required;
        },
        setRequired: function (required) {
          el.required = required;
        },
        getValue: function () {
          return el.value;
        },
        setValue: function (value) {
          el.value = value;
        },
        getLabelledBy: function () {
          return el.labelledBy;
        },
        setLabelledBy: function (labelledBy) {
          el.labelledBy = labelledBy;
        },
        getValues: function () {
          return el.values;
        },
        setValues: function (values) {
          el.values = values;
        },
        clear: function () {
          el.clear();
        }
      };
    }
  });

  var _fixFileUploadThumbnail = function ($fileUploadEl) {
    var componentPath = $fileUploadEl.parents("form.cq-dialog").attr("action");
    var $thumbnail = $fileUploadEl.find(THUMBNAIL_SELECTOR);
    $thumbnail.empty();
    $thumbnail.closest('coral-fileupload').removeClass(IS_FILLED_CLASS);
    $.ajax({
      url: componentPath + itemNodePath.replace('./', '/') + ".json",
      cache: false
    }).done(function (data) {
      var params = {};
      $fileUploadEl.find('[data-cq-fileupload-parameter]').each(function () {
        var $param = $(this);
        var paramName = $param.data('cq-fileupload-parameter');
        var name = $param.attr('name');
        name = name.substr(name.lastIndexOf("/") + 1);
        value = $param.val();
        params[paramName] = {
          name: name,
          value: value
        };
      })

      var imagePath;
      if (params.filereference && params.filereference.name) {
        imagePath = data[params.filereference.name];
      } else if (params.filename && params.filename.name) {
        imagePath = params.filename ? data[params.filename.name] : undefined;
      }

      if (!imagePath) {
        return;
      }

      _createThumbnailFromRendition(imagePath, $thumbnail);
      $fileUploadEl.find('[data-cq-fileupload-parameter="filereference"]')
        .val(imagePath)
        .attr('disabled', null);
    });
  }

  var _createThumbnailFromRendition = function (imagePath, $container, prefix) {
    if (!prefix) {
      prefix = WEB_RENDITION_PREFIX;
    }
    var self = this;
    var requestPath = imagePath + RENDITIONS_PATH + ".1.json";

    $.getJSON(requestPath).then(function (renditionsData) {
      var webRenditionPath = null;
      for (var prop in renditionsData) {
        if (renditionsData.hasOwnProperty(prop)) {
          if (prop.indexOf(prefix) === 0) {
            webRenditionPath = imagePath + RENDITIONS_PATH + "/" + prop;
            break; // Only one web rendition currently. Doing same as WCMRenditionPicker.java (2014-11-13)
          }
        }
      }
      _appendThumbnail(webRenditionPath ? webRenditionPath : imagePath, $container);

    }, function () {
      $container.append($("<p>" + "Expecting DAM asset to have renditions, but request to " + requestPath + " failed" + "</p>"));
    });
  };

  var _appendThumbnail = function (thumbnailPath, $container) {
    var ckParam = "?:ck=" + (new Date()).getTime(); // Cache killer
    var $thumbnail = $("<img/>", {
      "alt": thumbnailPath,
      "class": "cq-dd-image",
      "src": thumbnailPath + ckParam
    });

    $container.append($thumbnail);
    $container.closest('coral-fileupload').addClass(IS_FILLED_CLASS);
  };
})();