Issue Summary

A thumbnail wasn’t appeared in a comment field when an image file was dragged by going through the browser before dropping into the commend field (such as path A or path B in attached screenshots).
At that time, "XXX has been attached successfully to YYY." popup windows was displayed at the top-right of the browser and the file was appeared in the "Attachment" field.

The symptom didn’t occur (i.e. the thumbnail was appeared in the comment field as expected) when the file was directly dragged and dropped into the comment field (such as path C in the attached screenshot).

It occurred on both "Visual" and "Text" mode, and also both "Wiki" and "Text" renderer.

The symptom occurred on JSM 5.12.4, 5.12.11 and 5.17.0.

Steps to Reproduce

1. Open a comment field by clicking "Add a comment…"
2. Drag an image file by going through the browser other than the target comment field, and then drop it into the target comment field

Expected Results

The thumbnail of the image file is displayed in the target comment field and the file itself added into "Attachments" field.

Actual Results

The file was shown in "Attachments" field, but the thumbnail wasn't shown in the target comment field.

Workaround

No workaround to prevent the symptom.
After the occurrence of the symptom, we can manually write "...|thumbnail" in the comment field with "Text" mode.