To share a file, a user sends a message stanza including <file-sharing/> to the inteded recipient contact or group. The <file-sharing/> element MAY have a disposition attribute with a value of either attachment or inline. The <file-sharing/> element includes a <file/> metadata element as described in &xep0446; as well as a <sources/> element. The <sources/> element provides one or multiple sources that the receiving client may use to obtain the file.

It is RECOMMENDED that the file metadata specifies name, media-type, size and one or multiple hash elements as described in &xep0300;. The hash elements provide end-to-end file integrity and allow efficient caching and flexible retrieval methods.

The message MAY include a suitable fallback body. The fallback body MUST NOT include any information that is not also represented in <file-sharing/>. If the <sources/> element includes an <url-data/> element that can be represented as a single URL, adding a &xep0066; x-oob reference is RECOMMENDED for compatibility.

If the message has an empty body, it is RECOMMENDED to add a message processing hint, see &xep0334;, to indicate the message to be stored in message stores like &xep0313;.

On receive of a message including a <file-sharing/> element, the receiving entity SHOULD lookup in a local storage, whether the file with any of the proivded hashes has already been retrieved and is available. In that case no transfer needs to be initated and the cached file can be used instead.

If the file is not available locally, the file can be obtained by one of the sources listed in the <sources/> element. If further sources have been attached (as described in Attaching a source), the receiving entity may also try to obtain the file from any of those.

The receiving entity SHOULD NOT fetch the file without prior user interaction if the disposition attribute is set to attachment. The receiving entity MAY fetch the file without prior user interaction otherwise, but when doing so, user's privacy, security (see Security Considerations) and network constraints should be considered.

When the source is an <url-data/> element as described in &xep0103;, the receiving entity MAY obtain the file by downloading it from the specified URL. If the URL uses HTTP or HTTPS and additional HTTP request information as specified in &xep0104; is provided, the receiving entity SHOULD use such information when obtaining the file. When sending and receiving files using <url-data/>, it is RECOMMENDED to prefer secure protocols (e.g. HTTPS, FTPS). Please read security considerations when implementing support for insecure URLs.

When the source is a <jingle-pub/> element as described in &xep0358;, the receiving entity MAY obtain the file using the protocol described in &xep0358;. If a <hash/> is provided, the receiving entity MAY obtain the file by requesting it as described in &xep0234;.

If sources of any other type are provided, clients MAY attempt to obtain the files from such sources. The details of obtaining such file are out of scope of this document.

The intended method to provide or display a file to the user depends on the disposition attribute, <media-type/> and file type support of the receiving entity:

• If the disposition attribute is set to attachment, no <media-type/> is provided or the <media-type/> indicates that the file can not be displayed inline, i.e. when the media type is application/octet-stream, the receiving entity SHOULD NOT display the file inline and instead offer to download it or save it on the user's file system.
• If the disposition attribute is set to inline, the receiving entity SHOULD attempt to display the file inline. When displaying the file inline fails, the receiving entity SHOULD indicate to the user that the file was meant to be displayed inline but that the file type was not supported for inline display and offer to open the file using an external viewer or to download the file or save it to the user's file system instead.
• If no disposition attribute is set and the <media-type/> of the shared file indicates that it can be displayed inline, the receiving entity MAY display the file inline. When displaying the file inline fails, the receiving entity SHOULD NOT display any error and instead offer to open the file using an external viewer or to download the file or safe it to the user's file system.

TODO: The following section relies on &xep0367;, however other methods to attach information to another message like the recently proposed &xep0422; might be suitable here as well. This is to be clarified before advancing to Draft.

After a user shared a file using one entity and another entity in the conversation obtained it or found it in its local storage, that entity MAY announce that the file is now available with an additional source. This increases availability of the file in case the sender goes offline before all the intended recipients were able to fetch the file. It also allows for peer-to-peer file distribution in group chats.

The entity MUST NOT announce itself as an additional source before verifying that all hashes provided match the hash of the file. If no hashes are provided, the entity SHOULD NOT announce itself as an additional source.

The attaching itself is performed by sending a message including a <sources> element with further sources using the protocol described in &xep0367;.

Depending on the lifetime of the newly attached source, it may be useful to add a message processing hint, see &xep0334;, to indicate the message to be stored in message stores like &xep0313;.

The ®ISTRAR; includes 'urn:xmpp:sfs:0' in its registry of protocol namespaces (see &NAMESPACES;).

• urn:xmpp:sfs:0