silverstripe-userforms/code/model/editableformfields/EditableFileField.php

<?php

/**
 * Allows a user to add a field that can be used to upload a file.
 *
 * @package userforms
 */

class EditableFileField extends EditableFormField {
	
	private static $singular_name = 'File Upload Field';
	
	private static $plural_names = 'File Fields';

	private static $has_one = array(
		'Folder' => 'Folder' // From CustomFields
	);

	/**
	 * @return FieldList
	 */
	public function getCMSFields() {
		$fields = parent::getCMSFields();
		
		$fields->addFieldToTab(
			'Root.Main',
			TreeDropdownField::create(
				'FolderID',
				_t('EditableUploadField.SELECTUPLOADFOLDER', 'Select upload folder'),
				'Folder'
			)
		);

		return $fields;
	}

	public function getFormField() {
		$field = FileField::create($this->Name, $this->Title);

		// filter out '' since this would be a regex problem on JS end
		$field->getValidator()->setAllowedExtensions(
			array_filter(Config::inst()->get('File', 'allowed_extensions'))
		);

		$folder = $this->Folder();
		if($folder && $folder->exists()) {
			$field->setFolderName(
				preg_replace("/^assets\//","", $folder->Filename)
			);
		}

		if ($this->Required) {
			// Required validation can conflict so add the Required validation messages
			// as input attributes
			$errorMessage = $this->getErrorMessage()->HTML();
			$field->setAttribute('data-rule-required', 'true');
			$field->setAttribute('data-msg-required', $errorMessage);
		}

		return $field;
	}
	
	
	/**
	 * Return the value for the database, link to the file is stored as a
	 * relation so value for the field can be null.
	 *
	 * @return string
	 */
	public function getValueFromData() {
		return null;
	}
	
	public function getSubmittedFormField() {
		return new SubmittedFileField();
	}


	public function migrateSettings($data) {
		// Migrate 'Folder' setting to 'FolderID'
		if(isset($data['Folder'])) {
			$this->FolderID = $data['Folder'];
			unset($data['Folder']);
		}

		parent::migrateSettings($data);
	}
}