Attachments: Fixed drag into editor in Chrome

Seemed to be chrome specific from testing.
Required editors to have preventDefault called on dragover.
Tested in Chrome, FF, & Safari.
Tested in both editors, and re-tested text/image drop to ensure still
works.

Fixed #4975

resources/js/markdown/codemirror.js

@@ -44,6 +44,10 @@ export async function init(editor) {
                 editor.actions.insertClipboardImages(clipboardImages, event.pageX, event.pageY);
             }
         },
+        // Handle dragover event to allow as drop-target in chrome
+        dragover: event => {
+            event.preventDefault();
+        },
         // Handle image paste
         paste: event => {
             const clipboard = new Clipboard(event.clipboardData || event.dataTransfer);