[SalesForce] Send chatter files with REST

When I upload a file from the directory (and save it as an attachment) and send it with REST webservices, it works great, but when I get a file from Salesforce (from chatter for example) it doesn't work…

I tried to change the type of the file, but it still doesn't work.

I also tried once to get a file from attachment and attach this one to an object and tried to send it after but it didn't work.

I think it's a problem of type (blob, base 64..)

What can cause this problem?

I have this error:

System.CalloutException: You have uncommitted work pending. Please commit or rollback 
before calling out Erreur dans l'expression '{!createFolder}' du composant <apex:page> dans
page signeco_creatasigneco Class.Signeco_Connector.createFolder: line 125, column 1 
Class.CreateSignecoController.createFolder: line 350, column 1 the firstg concern this    one:
HTTPResponse res = http.send(req); The second: cf Signeco_Connector.createFolder( folder,
fileToSignEnv, destinataires); 

// the webservice call I use REST webservices.

Here is my code the problem is that th id of the attachment is null, i use to store information , so i cant put an id, do i have to inser it and update after ???

// Visualforce page

// controlleur methode

public PageReference createFolder() {
fileToSign = ([SELECT Id, Type, CreatedDate, Body, Title, LinkUrl, ContentData, ContentSize, CreatedById, CreatedBy.FirstName, CreatedBy.LastName, ParentId, Parent.Name, ContentFileName, ContentType From FeedItem where Type = 'ContentPost' and
ParentId=: folder.id order by createddate, ID DESC Limit 1]);

fileToSignEnv.body = fileToSign.ContentData;
fileToSignEnv.ParentId = folder.id;
fileToSignEnv.name= 'Test';

fileToSign= null;


//fileToSignEnv.body = Encodingutil.base64Decode(String.valueOf(fileToSignEnv.body));
insert fileToSignEnv;
ApexPages.addmessage(new ApexPages.message(ApexPages.severity.INFO,'Le body de chatter!'+fileToSignEnv.body));
   if (folder.Id == null ||  fileToSignEnv.Id == null ||destinataires.get(0) == null) {  ApexPages.addmessage(new ApexPages.message(ApexPages.severity.INFO,'Veuillez renseigner les champs obligatoires'+fileToSignEnv.Id ));}
   else { System.debug('voici le doc'+fileToSign.ContentData);
          cf = Signeco_Connector.createFolder(folder,  fileToSignEnv, destinataires);
          fileToSign= null;
        ApexPages.addmessage(new ApexPages.message(ApexPages.severity.INFO,'Your folder has been send to be signed!'));
        System.debug('voici l\'affichage'+cf);
        Map<String, Object> resultJSON = (Map<String, Object>)JSON.deserializeUntyped(cf);
        folder.SignecoId__c = (String) resultJSON.get('reference');
       // cf = '###  '+resultJSON;
        update folder; 
   }
   return null;
}

// REST webservice

public static String createFolder(Signeco__c folder, Attachment docToSign, List dest) {
HttpRequest req = new HttpRequest();
//req.setEndpoint(ENDPOINT+'createRecord?arg0=1');
req.setMethod('POST');

    Blob userNameValue = Blob.valueOf(USERNAME);
    Blob passwordValue = Blob.valueOf(PASSWORD);
    req.setHeader('X-OTC-Auth-Ident', EncodingUtil.base64Encode(userNameValue));
    req.setHeader('X-OTC-Auth-Password', EncodingUtil.base64Encode(passwordValue));
    req.setHeader('Content-Type','application/x-www-form-urlencoded');

System.debug('On est dans la classe conencteurs'+docToSign.body);
List docs = new List();
docs.add(new Document(docToSign));

    List<Recipient> recps = new List<Recipient>();

    for (Destinataires_Signeco__c d : dest) {
        recps.add(new Recipient('PERSONAL',d.Email__c));
    }

    JSONGenerator gen = JSON.createGenerator(true);
    gen.writeStartObject();
    gen.writeStringField('service', 'Keynectis SIGNECO Service');
    gen.writeStringField('subject', folder.Name);
    gen.writeStringField('comment ', folder.description__c);
    gen.writeObjectField('documents', docs);
    gen.writeObjectField('recipients', recps);
    gen.writeEndObject();
    req.setBody('arg0='+ EncodingUtil.urlEncode(gen.getAsString(),'UTF-8.'));



   // req.setEndpoint(ENDPOINT+'createRecord'+ EncodingUtil.urlEncode(gen.getAsString(),'UTF-8.'));

    req.setEndpoint(ENDPOINT+'createRecord');
    Http http = new Http();
    HTTPResponse res = http.send(req);

   return res.getBody();

}