controller.php

/**
 * Upload files.
 *
 * @return void
 */
public function uploadAction() {
	$overwriteExistingFiles = TRUE;

	$data = array();
	$namespace = key($_FILES);
	$targetFalDirectory = '1:/_temp_/';

	// Register every upload field from the form:
	$this->registerUploadField($data, $namespace, 'myimage', $targetFalDirectory);

	// Initializing:
	/** @var \TYPO3\CMS\Core\Utility\File\ExtendedFileUtility $fileProcessor */
	$fileProcessor = \TYPO3\CMS\Core\Utility\GeneralUtility::makeInstance('TYPO3\\CMS\\Core\\Utility\\File\\ExtendedFileUtility');
	$fileProcessor->init(array(), $GLOBALS['TYPO3_CONF_VARS']['BE']['fileExtensions']);
	$fileProcessor->setActionPermissions(array('addFile' => TRUE));
	$fileProcessor->dontCheckForUnique = $overwriteExistingFiles ? 1 : 0;

	// Actual upload
	$fileProcessor->start($data);
	$result = $fileProcessor->processData();

	// Do whatever you want with $result (array of File objects)
}

/**
 * Registers an uploaded file for TYPO3 native upload handling.
 * 
 * @param array &$data
 * @param string $namespace
 * @param string $fieldName
 * @param string $targetDirectory
 * @return void
 */
protected function registerUploadField(array &$data, $namespace, $fieldName, $targetDirectory = '1:/_temp_/') {
	if (!isset($data['upload'])) {
		$data['upload'] = array();
	}
	$counter = count($data['upload']) + 1;

	$keys = array_keys($_FILES[$namespace]);
	foreach ($keys as $key) {
		$_FILES['upload_' . $counter][$key] = $_FILES[$namespace][$key][$fieldName];
	}
	$data['upload'][$counter] = array(
		'data' => $counter,
		'target' => $targetDirectory,
	);
}

fluid-template.html

<f:form action="upload" enctype="multipart/form-data">
	<f:form.upload name="myimage" />
	<f:form.submit value="GO" />
</f:form>