system/HTTP/Files/FileCollection.php
protected function populateFiles()
	{
		if (is_array($this->files))
		{
			return;
		}

		$this->files = [];

		if (empty($_FILES))
		{
			return;
		}

		$files = $this->fixFilesArray($_FILES);

		foreach ($files as $name => $file)
		{
			$this->files[$name] = $this->createFileObject($file);
		}
	}
protected function populateFiles()
	{
		if (is_array($this->files))
		{
			return;
		}

		$this->files = [];

		if (empty($_FILES))
		{
			return;
		}

		$files = $this->fixFilesArray($_FILES);

		foreach ($files as $name => $file)
		{
			$this->files[$name] = $this->createFileObject($file);
		}
	}
Taking information from the $_FILES array, it creates an instance
of UploadedFile for each one, saving the results to this->files.

Called by files(), file(), and hasFile()