Symptoms

Users are unable to attach images via html editor.
After clicking on the "Insert/Edit image" option on the HTML editor, user prompted with the "Insert/Modify Image" modal window. In which, the user could browse an image from the desktop, and click on the attach button. Then the attach process happening forever as shown in the below screenshot.

Release

Issue can occur on any supported release. Specifically, the issue was observed after London upgrade.

The out of the box provided, "html_insert_image_dialog" UI Page (https://InstanceName.service-now.com/nav_to.do?uri=sys_ui_page.do?sys_id=c20555420a0a0b1e00806f8e72e98c6a) is responsible for rendering the "Insert/Modify image" popup window.
Seems to this UI page was customized on the affected instance and hence updates on the were file was skipped during London upgrade.
Due to these skipped updates, the attach image function not working as expected.

Please open the skipped upgrade record for "html_insert_image_dialog" UI Page as per below steps and then revert it to base system version, this should resolve the reported issue. If there are any customization, you might need to merge it with out of the box version carefully.

Log on to the affected instance
Navigate to "System Diagnostics=>Upgrade History"
Select and open the "Jakarta to London" upgrade history record
Open the "skipped changes to Review" section tab
Filter for the "html_insert_image_dialog" UI Page with "File name = sys_ui_page_c20555420a0a0b1e00806f8e72e98c6a"
Open the record and click on "Revert to Base System" or merge the updates with your customization carefully.