问MVC3 -值不能为空或空。参数名称: contentType

EN

@foreach (var item in Model) 
<td>
      <img src="@Url.Action("GetImage", "ProductCategoryL2", new { id =  
      @item.SubProductCategoryID })" alt="" height="100" width="100" /> 
    </td>

@using (Html.BeginForm("ProductCategoryL2", "GetImage", FormMethod.Post, 
new { @enctype = "multipart/form-data" }))
{
  <div class="editor-field">
     <img src="@Url.Action("GetImage", "ProductCategoryL2", new { id = 
      @Model.SubProductCategoryID })" alt="" /> 
      @Html.ValidationMessage("Picture1", "*")
     <input type="file" name="Image" size="23"/>
  </div>
}

public FileContentResult GetImage(int id)
    {
        var product = db.SubProductCategory2.First(x => 
        x.SubProductCategoryID == id);
        return File(product.Picture1, product.ImageMimeType); //Value cannot be null or empty. Parameter name: contentType 
    }

   [ScaffoldColumn(false)]
   public string ImageMimeType { get; set; }