Since Jakarta, users can add an attachment which is not of image type (e.g. Excel) when using the "Insert/edit image" button in an HTML field with the TinyMCE Editor.
In Istanbul the operation was not allowed and a message alerts the attachment is not a valid image type, blocking the file to get added to the record.
Steps to Reproduce
1. Open an incident record.
2. Add a new field of type HTML into the form.
3. Click on the "Insert/edit image" button in the TinyMCE HTML editor toolbar.
4. Leave "Type" as attachment, click on the paperclip icon and add a file which is not an image, e.g. an Excel file.
No error messages is prompted about the file type, and the non-image file still gets attached to the record.
Related Problem: PRB1210235