Improve attachment upload methods (#30513)

* Use dropzone to handle file uploading for all cases, including pasting
and dragging
* Merge duplicate code, use consistent behavior for link generating

Close #20130

---------

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/js/features/comp/EditorMarkdown.js

@@ -1,4 +1,6 @@
-import {triggerEditorContentChanged} from './Paste.js';
+export function triggerEditorContentChanged(target) {
+  target.dispatchEvent(new CustomEvent('ce-editor-content-changed', {bubbles: true}));
+}
 
 function handleIndentSelection(textarea, e) {
   const selStart = textarea.selectionStart;