POST /services/data/v34.0/chatter/feed-elements HTTP/1.1
Authorization: OAuth 00DRR0000000N0g!...
User-Agent: Jakarta Commons-HttpClient/3.0.1
Host: instance_name
Content-Length: 845
Content-Type: multipart/form-data; boundary=a7V4kRcFA8E79pivMuV2tukQ85cmNKeoEgJgq
Accept: application/json
--a7V4kRcFA8E79pivMuV2tukQ85cmNKeoEgJgq
Content-Disposition: form-data; name="json"
Content-Type: application/json; charset=UTF-8
{
   "body":{
      "messageSegments":[
         {
            "type":"Text",
            "text":"Please accept this receipt."
         }
      ]
   },
   "capabilities":{
      "content":{
         "description":"Receipt for expenses",
         "title":"receipt.pdf"
      }
   },
   "feedElementType":"FeedItem",
   "subjectId":"005RR000000DmOb"
}

--a7V4kRcFA8E79pivMuV2tukQ85cmNKeoEgJgq
Content-Disposition: form-data; name="feedElementFileUpload"; filename="receipt.pdf"
Content-Type: application/octet-stream; charset=ISO-8859-1

...contents of receipt.pdf...

--a7V4kRcFA8E79pivMuV2tukQ85cmNKeoEgJgq--

var item =
{
    "body":
    {
        "messageSegments" : segments
    },
    "feedElementType" : "FeedItem",
    "subjectId" : "me"
};

if(fileData)
{
    item.capabilities =
    {
        "content" :
        {
            "description": "File attachment from Clienteling",
            "title": "Some File"
        }
    };

    var data = new FormData();
    data.append("json", JSON.stringify(item));
    data.append("feedElementFileUpload", fileData);

    var req = new XMLHttpRequest();

    req.addEventListener("load", function(event)
        {
            success(req);
        }, false);
    req.addEventListener("error", fail, false);

    req.open("POST", self.base + "/feed-elements", true);
    req.setRequestHeader("Authorization", self.bearer);
    req.send(data);
}

-----------------------------18721583984357993421560304724
Content-Disposition: form-data; name="json"

{"body":{"messageSegments":[{"type":"Text","text":"This is a file post."}]},"feedElementType":"FeedItem","subjectId":"me","capabilities":{"content":{"description":"File attachment","title":"Some File"}}}
-----------------------------18721583984357993421560304724
Content-Disposition: form-data; name="feedElementFileUpload"; filename="39191-ki1RNjpg-EdDM.jpg"
Content-Type: image/jpeg

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SNIP
-----------------------------18721583984357993421560304724--

<?xml version="1.0" encoding="UTF-8"?><Errors><Error><errorCode>MISSING_ARGUMENT</errorCode><message>Missing required &apos;subjectId&apos; parameter.</message></Error></Errors>