How to work with an Input File in an HTML form?

HTML: One file

<input type="file"  />

HTML: Multiple Files

• The HTML

<input type="file" multiple />

• Result

How to set the default file value?

• from the operating system:
  • This is not possible to set the default file.
  • It would then be a security issue because you could steal files from user's computers.
• generated: you can create and set a file object with a generated content

HTML: All files under a directory

When using the webkitdirectory attribute (supported on all browsers ²⁾), you can upload all files present under a directory and its children.

Example:

• The HTML

<input type="file" webkitdirectory  />

Note that after choosing your directory, you will get the following warning dialog

• The result:

HTML: Disabled

<input type="file" disabled  />

HTML: How to select only a certain type of file

The accept attribute specifies the file types accepted (mime and/or file extension)

Example for image and text file

<input type="file" accept="image/*,.txt"/>

HTML: How to change the description of the button

Changing the description of the input file cannot be done via a standard attribute.

It's also not possible to disable the native default browser rendering of a input file (the appearance css property is not working).

The trick is to create a hidden input file with a visible label that refers to it via the for attributes.

• The html

<label for="my-input-file-id" class="btn btn-primary">Select an Image</label>
<input id="my-input-file-id" style="visibility:hidden;" type="file">

• Result:

Javascript: How to manipulate the chosen file

To see examples of how Javascript can take and handle the file chosen, see the dedicated fileApi page

Javascript: How to create and send with FormData

When a file is send with multipart form data, the file is added in the formdata as a File object

Example:

• The form (Return false to prevent the default action of the submit event - ie to not get a 404)

<form onsubmit="handleSubmit(this); return false;">
<input type="file" name="myfile"/>
<br>
<input name="button" type="submit" value="Choose a file and see the data type" />
</form>

• The event handler that will handle the submit event and loop over the entries iterator

handleSubmit = (form) => {
  // build the formData object
  let formData =  new FormData(form);
  // retrieve the entries in a entries variable of iterator type
  let entries = formData.entries();
  // loop over the iterator
  let result = entries.next();
  while (!result.done) {
    console.log("Input: Name: "+result.value[0]+", value: "+result.value[1].constructor.name);
    result = entries.next();
  }
}

• Result: