Change dragover to dragenter

Dragenter is fired only once when the drag first touches the page, whereas dragover fires continuously.

As we’re only toggling the visbility of the dropzone, dragenter will suffice for our needs.

packages/govuk-frontend/src/govuk/components/file-upload/file-upload.mjs

@@ -121,7 +121,7 @@ export class FileUpload extends GOVUKFrontendComponent {
     this.$wrapper.addEventListener('drop', this.onDragLeaveOrDrop.bind(this))
 
     // When a file is dragged over the page (or dragged off the page)
-    document.addEventListener('dragover', this.onDragOver.bind(this))
+    document.addEventListener('dragenter', this.onDragEnter.bind(this))
     document.addEventListener('dragleave', this.onDragLeaveOrDrop.bind(this))
   }
 
@@ -176,8 +176,14 @@ export class FileUpload extends GOVUKFrontendComponent {
   /**
    * When a file is dragged over the container, show a visual indicator that a
    * file can be dropped here.
+   *
+   * @param {DragEvent} event - the drag event
    */
-  onDragOver() {
+  onDragEnter(event) {
+    // Check if the thing being dragged is a file (and not text or something
+    // else), we only want to indicate files.
+    console.log(event)
+
     // eslint-disable-next-line
     // @ts-ignore
     this.$wrapper.classList.add('govuk-file-upload-wrapper--show-dropzone')