Attachments that are added using the xmldoc-update-plugin in xwiki-platform-help-ui are added without mime type in contrast to those that are defined inline in the XML. The effect is that, e.g., in the list of all attachments, they cannot be found when filtering by mime type. It seems they still somehow have a mime type that is displayed in the list. For the attached screenshot, I re-uploaded the "Administration.mp4"-video as attachment to the "Main" page. The "Typ"-column contains the same content for both locations, but only the re-uploaded video is found. For images I can also reproduce that those inline in the XML with mime type are found while those attached using the xmldoc-update-plugin are not found.