Why Does File Upload Not Work Without the Enctype Property

Why does file upload not work without the enctype property?

The "multipart/form-data" enctype is specified by RFC 1867 which you can review here for more of a technical overview.

In HTML forms, data is represented as several fields. When using multipart/form-data as the enc type, the browser sends the form fields as a series of "parts" which each have a content-type header to describe the type of data stored in the part. This content-type is usually set to "text/plain" for normal form fields. This content-type is only sent by the browser when the multipart/form-data enctype is used.

For input elements of type "file", the content type is "application/octet-stream" or something similar which indicates to the server side software that the contents of the field are not typical plaintext but are instead the contents of a file and should be handled differently.

The reason input elements of type "file" do not work whenever "multipart/form-data" is not used is due to the fact that the server has no way of identifying that the contents of the field are any different from a normal text field (since the browser does not send the content-type unless multipart/form-data is used) so it handles the contents of the field as normal text. When the proper enctype is used and the server can properly identify what type of data the field contains, the server knows to handle the contents of the field as file data instead of text and can process it properly.

What does enctype='multipart/form-data' mean?

When you make a POST request, you have to encode the data that forms the body of the request in some way.

HTML forms provide three methods of encoding.

• application/x-www-form-urlencoded (the default)
• multipart/form-data
• text/plain

Work was being done on adding application/json, but that has been abandoned.

(Other encodings are possible with HTTP requests generated using other means than an HTML form submission. JSON is a common format for use with web services and some still use SOAP.)

The specifics of the formats don't matter to most developers. The important points are:

• Never use text/plain.

When you are writing client-side code:

• use multipart/form-data when your form includes any <input type="file"> elements
• otherwise you can use multipart/form-data or application/x-www-form-urlencoded but application/x-www-form-urlencoded will be more efficient

When you are writing server-side code:

• Use a prewritten form handling library

Most (such as Perl's CGI->param or the one exposed by PHP's $_POST superglobal) will take care of the differences for you. Don't bother trying to parse the raw input received by the server.

Sometimes you will find a library that can't handle both formats. Node.js's most popular library for handling form data is body-parser which cannot handle multipart requests (but has documentation that recommends some alternatives which can).

---

If you are writing (or debugging) a library for parsing or generating the raw data, then you need to start worrying about the format. You might also want to know about it for interest's sake.

application/x-www-form-urlencoded is more or less the same as a query string on the end of the URL.

multipart/form-data is significantly more complicated but it allows entire files to be included in the data. An example of the result can be found in the HTML 4 specification.

text/plain is introduced by HTML 5 and is useful only for debugging — from the spec: They are not reliably interpretable by computer — and I'd argue that the others combined with tools (like the Network Panel in the developer tools of most browsers) are better for that).

HTML Form With Enctype Breaks $_POST Data

To anyone who suffers the same problem, check out this page:

WAMP Uploading large File

The question gives the values you need to change in your php.ini file to allow larger file uploads (that's what was breaking the form).

HTML Form File Uploads doesn't upload file

you forgot to mention the enctype="multipart/form-data"

<form action="upload_handler.php" enctype="multipart/form-data" method="post">
    Select image to upload:
    <input type="file" name="fileToUpload" id="fileToUpload">
    <input type="submit" value="Upload" name="submit">
</form>

---

Related Topics