linux - Thinkphp文件上传错误提示：不存在的图像文件，本地测试不会这样的，放到服务器才会这样，为什么？

<code>$(".upavater").change(function(){
  var fdpic=new FormData();
  var pic=$("#upavater").get(0).files[0];
  fdpic.append("pic",pic);
$.ajax({  
          url: avaterurl,  
          type: 'POST',  
          data: fdpic,  
          //async: false,  
          cache: false,  
          enctype: 'multipart/form-data',
          contentType: false, 
          processData: false
          success:function(d){colsole.log(d);}
      })
})</code>

<code>public function avater(){
        $upload = new \Think\Upload();// 实例化上传类
        $upload->maxSize   =     3145728 ;// 设置附件上传大小3M
        $upload->exts      =     array('jpg', 'gif', 'png', 'jpeg');// 设置附件上传类型
        $upload->rootPath  =     "./Public/avater/"; // 设置附件上传根目录
        $upload->subName  =     array('date','Ym');
        $info   =   $upload->upload();
        if($Info){echo "上传成功";}else{echo "上传失败";}
        }</code>

<code>$(".upavater").change(function(){
  var fdpic=new FormData();
  var pic=$("#upavater").get(0).files[0];
  fdpic.append("pic",pic);
$.ajax({  
          url: avaterurl,  
          type: 'POST',  
          data: fdpic,  
          //async: false,  
          cache: false,  
          enctype: 'multipart/form-data',
          contentType: false, 
          processData: false
          success:function(d){colsole.log(d);}
      })
})</code>

<code>public function avater(){
        $upload = new \Think\Upload();// 实例化上传类
        $upload->maxSize   =     3145728 ;// 设置附件上传大小3M
        $upload->exts      =     array('jpg', 'gif', 'png', 'jpeg');// 设置附件上传类型
        $upload->rootPath  =     "./Public/avater/"; // 设置附件上传根目录
        $upload->subName  =     array('date','Ym');
        $info   =   $upload->upload();
        if($Info){echo "上传成功";}else{echo "上传失败";}
        }</code>