When attaching images or files to a post or private message, after picking the file to attach you do not get a dialog box asking for the attachment comment if certain attachment settings are set to zero (allow unlimited). Changing the following values from 0 in ACP Attachment settings solved the issue. Either this is a bug or the documentation on the page saying this is allowed is incorrect.

Put non-zero values into the following fields solved the problem:

• Total attachment quota
• Maximum file size
• Maximum file size messaging