image validation error : The profile picture must be an image

I am new to Laravel. I’m using Laravel6.0.
I got the error message `The profile picture must be an image.’ but I don’t find any solutions.
Could you help me?

This is my Controller, Request , create.blade.php

 public function rules()
{
    return [
        'user_id'=>'required',
        'title' => 'required|string|max:255',
        'continent' => 'required|string',
        'picture'=>'required|image|max:1000',
        'country' => 'required|string',
        'costs'=>'required|numeric',
        'span'=>'required|numeric',
        'content'=>'required|string|max:250',
    ];

  namespace AppHttpControllers;

use IlluminateHttpRequest;
use AppHttpRequestsContentRequest;
use AppContent;

class ContentController extends Controller



public function store(ContentRequest $request)
  {
      if($request->validated()){
          $content =new Content;
          
          $filename = $request->file('image')->store('public/image');
          $content->picture = basename($filename);

          $content->user_id = $request->user_id;
          $content->content = $request->content;
          $content->title = $request->title;
          $content->span = $request->span;
          $content->continent = $request->continent;
          $content->country = $request-> country;
          $content->costs = $request->costs;
          
          $content->save();
      }
      
      return redirect('/');
      
  }

create.blade.php



@extends('layouts.app')
@section('content')
<div class="card">
  <div class="card-body">
     <form action="{{route('content.store')}}" method="post"　enctype="multipart/form-data" >
          @csrf
        <div class="form-group">
          <label for="exampleFormControlInput1">タイトル</label>
          <input type="text" class="form-control" id="exampleFormControlInput1" placeholder="旅のタイトル" name="title" value="{{old('title')}}" >
        </div>
        <div class="form-group">
          <label for="exampleFormControlSelect1">大陸名</label>
          <select class="form-control" id="exampleFormControlSelect1" name="continent"  value="{{old('continent')}}">
            <option>アジア</option>
            <option>北アメリカ</option>
            <option>中南米</option>
            <option>ヨーロッパ</option>
            <option>アフリカ</option>
            <option>オセアニア</option>
          </select>
        </div>
        <div class="form-group">
          <label for="exampleFormControlInput1">国名</label>
          <input type="text" class="form-control" id="exampleFormControlInput1" placeholder="日本" name="country" value="{{old('country')}}">       
        </div>
        <div class="form-group">
          <label for="exampleFormControlSelect1">滞在期間</label>
          <select class="form-control" id="exampleFormControlSelect1" name="span" placeholder="滞在期間を選択"  >
            <option>1</option>
            <option>2</option>
            <option>3</option>
            <option>4</option>
            <option>5</option>
            <option>6</option>
            <option>7</option>
            <option>8</option>
            <option>9</option>
            <option>10</option>
            <option>11</option>
          　<option>12</option>
          　<option>13</option>
          　<option>14</option>
          　<option>15</option>
          　<option>16</option>
          　<option>17</option>
          </select>
        </div>
        <div class="form-group">
          <label for="exampleFormControlSelect1">費用</label>
          <select class="form-control" id="exampleFormControlSelect1" name="costs" placeholder="金額を選択" value='{{old('costs')}}'>
            <option>10000</option>
            <option>30000</option>
            <option>50000</option>
            <option>70000</option>
            <option>100000</option>
            <option>150000</option>
            <option>200000</option>
            <option>250000</option>
            <option>300000</option>
          </select>
        </div>
        <div class="form-group">
          <label for="exampleFormControlFile1">写真</label>
          <input type="file" class="form-control-file" id="exampleFormControlFile1" name="picture" value="{{old('picture')}}">
          <div class="input-group-append">
              <button type="submit" class="btn btn-outline-secondary reset"><i class="fas fa-times fa-fw"></i>取消</button>
          </div>
        </div>
        <div class="form-group">
          <label for="exampleFormControlTextarea1">コンテンツ</label>
          <textarea class="form-control" id="exampleFormControlTextarea1" rows="5" name="content"  >{{old('content')}}</textarea>
        </div>
        <input type="hidden" name="user_id" value="{{Auth::id()}}">
        <button type="submit" class="btn btn-primary btn-lg btn-block">投稿</button>
      </form>
  </div>
</div>
@endsection

I tried

1, added enctype="multipart/form-data" in ‘create.blade.php’

2, added dd($request->all()) in my rules(),but i got the same error messages.