unknown format "base64" ignored in schema at path

I am working with this json:

{ 
     "type": "object", 
     "properties": { 
          "file": {  
               "type": "string" , 
               "format":"base64" , 
               "maxSize":"500000" , 
               "title":"Upload file" 
          } 
     } 
}

And I have this issue:

unknown format "base64" ignored in schema at path "#/properties/file"