This page presents a specific example of extending the base binary-posting specification to add new features. Similar extensions could be created for other types of media or other kinds of feature additions.
Here, I specify a way of posting pictures to Usenet which allows clients to create thumbnail displays of the pictures available in a newsgroup without needing to download the full pictures first (similar to the functionality you would get on the kind of web-based thumbnail display featured by services like Easynews). It also allows users to determine whether they already have a given picture (even under a different filename). In many picture newsgroups, reposting images under different filenames is a common complaint.
Currently, thumbnail posts are generally done by simply creating an image of thumbnails and posting it along with a series of pictures. The thumbnail image shows the filenames of the pictures, so a user can look through the Subject lines to find the posts containing the pictures he wants. Surely there must be a better way.
What we want from thumbnail posts is twofold. First, a way for a client to automatically determine that a post contains thumbnails. And second, a way to connect each thumbnail to the original, full-sized picture it represents.
Both goals can be acheived with MIME. We define a new MIME content-type, multipart/thumbnails. This type is similar to multipart/related.
The thumbnail post may contain an (optional) text part including descriptive text about the picture series it represents. Then, it contains a number of image parts (usually jpeg, but this is not required), one for each thumbnail.
(Existing MIME-compliant software should treat an unknown multipart type as multipart/mixed, and should therefore still be somewhat usable. A post of this type will be of little utility to a client which does not understand it, but such a client would, at the least, still be able to present the text description to the user, and may even display the thumbnail images.)
The following parameter is defined for multipart/thumbnails.
If the description is in HTML, or an HTML part within a multipart/alternative part, it can refer to the thumbnail parts in the same manner as with multipart/related, and a client can choose to render it as the thumbnail display if it wishes to do so. However, it need not do so, in particular if displaying thumbnails for multiple sets of images at once.
Each image entity within the multipart/thumbnails message will have several important pieces of information in its header.
Given the above, a client can find all the thumbnail posts in a group. It can then display a screen of thumbnail images, including filenames and captions, along with text descriptions for any sets which include one in the thumbnail post. A user could then click on one of the images to download the corresponding full post, since the client would have the message-id of that post.
A client could, therefore, show thumbnails of an entire picture group (assuming thumbnail posts are used in the group) without needing to download the group's overviews (headers) first.
For the full image posts, it is desirable to have a "link" to the corresponding thumbnail post, so a user who sees one of the pictures first can find the thumbnails for the set it goes with. In this case, it is not of great importance where the information is placed, so we can simply add a header to the post for it. So, a post of an image which has a corresponding thumbnail post can have an X-Thumbnail-Post header containing the message-id of the thumbnail post. Implementations must recognize this also as Thumbnail-Post (no X- prefix), which may be the specified form in the future, but for now, it must be generated with the prefix.
Note that this creates two posts which refer to each other by message-id. This will require that the client creates the message-ids for the posts, because that is the only way to know ahead of time what the ids will be. My suggestion, if a client wishes to use X-Thumbnail-Post, would be to post the full images first, having decided on a message-id to use for the thumbnail post. While posting the images, the client could build a list of message-ids corresponding to each image. Then, it could create the thumbnail post and send it. This procedure would be easiest even if the client does not wish to refer back to the thumbnail post from the full posts.
A very small number of servers do not allow client-supplied message-ids. I am aware of two such servers, and at least one of them does not even inform the client of the message-id it has decided to use for the post (it just silently changes a client-supplied id). This is unfortunate behavior which servers should not engage in (for reasons which go way beyond this specification), but unless they change, those servers would simply be considered unsuitable for posting of images. Even more unfortunately, there is no way for a client to automatically know that a server is going to discard client-supplied ids.