class DMSVersion extends DataObject {
	private static $db = array(
		'VersionNum' => 'Int',
		'CheckIn'  => 'Varchar(120)',
		'OriginalFilename' => 'Varchar(120)'
	);

	private static $has_one = array(
		'Document' => 'DMSDocument',
		'DMSFile' => 'File'
	);

	public function getCMSFields() {
		$fields = parent::getCMSFields();
		$fields->addFieldToTab('Root.Main', $UploadField = UploadField::Create('DMSFile', 'Upload version'));
		$UploadField->setAllowedFileCategories(array('doc', 'image'));
		$UploadField->setAllowedMaxFileNumber(1);
		$UploadField->setCanAttachExisting(false);
		$UploadField->setCanPreviewFolder( false );
		$UploadField->setFolderName(Config::inst()->get('DMSDocument', 'upload_dir'));
		$fields->addFieldToTab('Root.Main', new TextField('CheckIn', 'Checked In By'), 'DMSFile');
		
		$fields->removeByName('VersionNum');
		$fields->removeByName('OriginalFilename');

		return $fields;
	}

	public function onBeforeWrite() {
		trigger_error(basename($this->DMSFile()->Filename));
		if(!$this->VersionNum) {
			$version = $this->Document()->MaxVersion + 1;
			$this->VersionNum = $version;
			$this->Document()->MaxVersion = $version;
		}

		// store the original filename
		$this->OriginalFilename = basename($this->DMSFile()->Filename);
		return parent::onBeforeWrite();
	}

	public function onAfterWrite() {
		// Make this the current version
		$this->Document()->CurrentVersionID = $this->ID;
		$this->Document()->write();
		
		// change the filename
		$this->DMSFile()->setFilename($this->newFilename());
		$this->DMSFile()->write();
		return parent::onAfterWrite();
	}
	
	public function onBeforeDelete() {
		$this->DMSFile()->delete();
		return parent::onBeforeDelete();
	}
	
	public function newFilename() {
		return dirname($this->DMSFile()->Filename) . DIRECTORY_SEPARATOR . $this->ID . '.' . $this->DMSFile()->getExtension();
	}
}

trigger_error(basename($this->DMSFile()->Filename)) in onBeforeWrite gives me the filename generated in onAfterWrite

How can I get the original uploaded filename??

I ended up hacking onAfterWrite() by adding

$filename = Convert::raw2sql(basename($this->DMSFile()->Filename));
DB::query('UPDATE "DMSVersion" SET "OriginalFilename"=\''.$filename.'\' WHERE DMSVersion.ID=\''.$this->ID.'\'');