Attachments in the knowledge article are not visible in "kb_article_view" page of knowledge portal. But the attachments are visible in the articles when opened in native UI(non-portal) and also visible in "kb_article" page of Service portal.
In 'kb_article_view' page, there is a separate widget "Knowledge Attachments" to show the attachment of the article. This widget collects the data such as 'tableName', 'article_sys_id' from the widget "Knowledge Article Content(the widget which displays the article body)" widget. When the user is using the customized 'Knowledge Article Content' widget and the 'tableName' or the 'article_sys_id' is not defined in the same custom widget, the attachment wouldn't be visible.
In order to resolve the issue, declare and defined the tableName/article_sys_id in the custom 'Knowledge Article Content' widget.