form 表单的text/plain 编码方式

form表单的enctype属性可以设置表单提交时表单内容的编码方式，可用的编码方式有：

  enctype

When the value of the method attribute is post, this attribute is the MIME type of content that is used to submit the form to the server. Possible values are:

• application/x-www-form-urlencoded: The default value if the attribute is not specified.
• multipart/form-data: Use this value if you are using an <input>  element with the type attribute set to "file".
• text/plain (HTML5)

其中， text/plain 很少使用，其编码方式如下：