什么是HTML中的多部分形式数据？

multipart/form-data 是上传文件时必需的编码方式，1. 当表单包含文件输入时，因普通编码无法有效处理二进制数据，需使用该编码；2. 使用时需设置 form 标签的 enctype="multipart/form-data" 且 method="post"；3. 浏览器会将每个字段作为独立部分发送，用边界字符串分隔，使服务器能正确解析文本和文件；4. 适用于上传文件或同时提交文本与文件的场景，但纯文本表单应避免使用以提升效率。

Multipart form data in HTML refers to a way of encoding form data when you're submitting files along with text inputs. It's commonly used when you need to upload files (like images, documents, etc.) from an HTML form to a server.

Why multipart form data is needed

When a form includes file inputs (<input type="file">), regular form encoding (like application/x-www-form-urlencoded) can't handle binary data efficiently. That’s where multipart/form-data comes in. This encoding splits the form data into multiple parts — one for each form field — and sends them together in a single request, allowing both text and binary data (like files) to be transmitted safely.

How to use it in HTML

To enable multipart form data, you must set the enctype attribute of the <form></form> element to multipart/form-data:

<form action="/upload" method="post" enctype="multipart/form-data">
  <input type="text" name="username" placeholder="Your name">
  <input type="file" name="profile_pic">
  <button type="submit">Upload</button>
</form>

Key points:

• method="post" is required — file uploads don’t work with GET.
• enctype="multipart/form-data" tells the browser to format the data as multipart.
• Each field (including files) becomes a separate "part" in the request body.

What happens behind the scenes

When the form is submitted:

• The browser creates a unique boundary string to separate each part.
• Each form field is sent as a section, including its name and, for files, the filename and content type.
• Example of how data might look in the request:

Content-Type: multipart/form-data; boundary=----WebKitFormBoundaryABC123

------WebKitFormBoundaryABC123
Content-Disposition: form-data; name="username"

JohnDoe
------WebKitFormBoundaryABC123
Content-Disposition: form-data; name="profile_pic"; filename="photo.jpg"
Content-Type: image/jpeg

(binary file data here)
------WebKitFormBoundaryABC123--

Servers use this structure to parse both text fields and files correctly.

When to use it

Use multipart/form-data when:

• Uploading files (images, PDFs, etc.)
• Form contains both text and file inputs
• You need to send binary data

Don’t use it unnecessarily — for simple text forms, the default encoding is lighter and faster.

Basically, multipart form data makes file uploads possible in web forms by packaging all inputs — including binary files — into a single, well-structured HTTP request.

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