API upload did not contain a file part

Hello there

I am trying to upload text files into Box directly from Salesforce API callouts.

When I try to call your API endpoint, I get a 400 error with the message "API upload did not contain a file part".

here is my code , 

 public static List<String> FileSubmission(Id OpportunityId)
    {
        string authToken=GetAuthToken();
        
        set<id> cdId=new set<id>();
        List<ContentDocumentLink> cdl=[SELECT ContentDocumentId, LinkedEntityId FROM ContentDocumentLink WHERE LinkedEntityId =: OpportunityId];
        if(cdl.size()>0){
            for(ContentDocumentLink cdlList:cdl)
            {
                cdId.add(cdlList.ContentDocumentId);
            }
        }
        List<ContentVersion> cv=[SELECT Title,VersionData,FileType FROM ContentVersion WHERE ContentDocumentId IN: cdId order by CreatedDate desc limit 1];
        String fileName=cv[0].Title;
        system.debug(cv[0].Title);
        List<string> str=new List<string>();
        str.add(fileName);
         System.JSONGenerator gen = System.JSON.createGenerator(true);
            gen.writeStartObject();
            gen.writeStringField('name', fileName);
            gen.writeFieldName('parent');
            gen.writeStartObject();
            gen.writeStringField('id','0');
            gen.writeEndObject();
            gen.writeEndObject();
            
            String jsonbody = gen.getAsString(); 
        
        if(cv.size()>0){
            String targetURL ='https://upload.box.com/api/2.0/files/content';
            String separationString = '--------------------------363928481663217658578398';
            string fileContent = EncodingUtil.base64Encode(cv[0].VersionData);
             
            
          
            String bodyPayload = '\n\n'+separationString + '\n Content-Disposition: form-data; name="attributes"\n\n'+jsonbody+'\n'+separationString+'\n content-disposition: form-data; name:"files"; filename="'+fileName+'"\n content-type: text/plain; \n\n'+fileContent+'\n'+separationString;
    
            System.debug('bodyPayload: '+bodyPayload);
            
            
           
            
            Http http = new Http();
            HttpRequest req = new HttpRequest();
            req.setHeader('Content-Type', 'multipart/form-data; boundary='+separationString);
            req.setHeader('Content-Length', String.valueOf(fileContent.length()));
            req.setHeader('Authorization', 'Bearer '+authToken);
            req.setMethod('POST');
            req.setEndpoint(targetURL);
       
            req.setBody(bodyPayload);
   
            system.debug('AuthToken-------------------' +authToken);
            HttpResponse response = http.send(req);
            system.debug('getError ----'+ response.getBody()) ;
            system.debug('response'+response);
            if(response.getStatusCode() == 200){
                
                 system.debug('success');
                return str;
            }

       }
        return null;
    }
            

here is my response, with picture,

access token = yVKXzm1Zas78Zz67q7lUDcJ4tNYAF8v

file name = SampleFileTEXTNOTEPAD

"code":"bad_request","help_url":"http://developers.box.com/docs/#errors","status":400,"message":"API upload did not contain a file part","type":"error"