What does it mean "Successfully converted file [name].csv" in sync?

I got error when running Bulk API import data, the error occurs in sync step. "There are no mapped fields in the file [name].csv"

Importing data format is application/json,  this is explicitly configured in JAVA source code:

======================================================================

HttpURLConnection conn = (HttpURLConnection) url.openConnection();

conn.setInstanceFollowRedirects(false);

conn.setRequestMethod(method.toString());

conn.setRequestProperty("Content-Type", "application/json");

conn.setRequestProperty("Authorization", _authToken);       

    if (method == Method.POST || method == Method.PUT)

     {

  conn.setDoOutput(true);

    final OutputStream os = conn.getOutputStream();

     os.write(body.getBytes());

     os.flush();

     os.close();

    }

=============================================================

Sync result /api/bulk/1.0/sync/id/result:

==================================
{
  "elements":
  [
   {
    "syncUri":"/sync/5923",
    "count":0,
    "severity":"information",
    "message":"Successfully converted file 2014-04-03T03-57-14_User576_4ed54606b5ca4fb1b34092e32fede84e.json.",
    "createdAt":"/Date(1396497434330)/"
   },
   {
    "syncUri":"/sync/5923",
    "count":0,
    "severity":"error",
    "message":"Successfully converted file 2014-04-03T03-50-10_User576_393cef6059224e638282526dad6dcd1b.csv.",
    "createdAt":"/Date(1396497434393)/"
   },
   {
    "syncUri":"/sync/5923",
    "count":0,
    "severity":"error",
    "message":"There are no mapped fields in the file 2014-04-03T03-57-14_User576_4ed54606b5ca4fb1b34092e32fede84e.csv.",
    "createdAt":"/Date(1396497434393)/"
   },
   {
    "syncUri":"/sync/5923",
    "count":0,
    "severity":"error",
    "message":"There was an error processing the import.",
    "createdAt":"/Date(1396497434377)/"
   },
   {
    "syncUri":"/sync/5923",
    "count":0,
    "severity":"warning",
    "message":"Trying to unlock file 2014-04-03T03-57-14_User576_4ed54606b5ca4fb1b34092e32fede84e.csv on import 5820 for user 576. File has been unlocked.",
    "createdAt":"/Date(1396497434393)/"
   },
   {
    "syncUri":"/sync/5923",
    "count":0,
    "severity":"information",
    "message":"Sync processed for sync 5923, resulting in Error status.",
    "createdAt":"/Date(1396497434393)/"
   }
  ],
  "total":6,
  "pageSize":1000,
  "page":1
}

=====================================================

Why it convert file in CSV format in sync step?  How to fix the issue?

Fred Sakr Do you have any comments?