HTML forms are a critical component of web interactivity, with file upload being one of the most common use-cases. File upload in HTML forms is facilitated by the <input type="file"> element. This element creates a browse button, allowing users to select a file from their local filesystem. The selected file is subsequently included as part of the form data upon form submission.

It's crucial to note that when a form includes file data, the enctype attribute of the form must be set to multipart/form-data. This ensures that the file data is correctly included in the form submission, with the data being split into multiple parts.

The uploaded file is processed by a server-side script, which could be written in a language like PHP or Python. This script is responsible for parsing the form data, extracting the uploaded file, and processing it as necessary. Here's an example of how a file upload might be handled in PHP:

<?php
$target_dir = "uploads/";
$target_file = $target_dir . basename($_FILES["myFile"]["name"]);
move_uploaded_file($_FILES["myFile"]["tmp_name"], $target_file);
?>

In this PHP script, the $_FILES superglobal array is used to access the uploaded file. The file is moved from a temporary directory to a target directory using the move_uploaded_file function. It's important to handle potential errors and security issues during this process, such as checking the file size, file type, and ensuring the file does not overwrite an existing file.