Trigger change event of the markdown textarea to allow Vue catch the programmatic changes.

Before	After

Here is what happens step by step.

• We paste an image
• We know that there is an attachment in the pasted thing
• We show a placeholder text, like {{foo}}
• We start uploading the file
• We programmatically change the value of the textarea after upload finished

At this point Vue doesn't catch the programmatically changed textarea value. So in the dropzone.js's paste handler I triggered change event of the textarea manually.

Fixes #38441 (closed)