add public function getMimeTable()

Return Extention/MIME Table (elFinderVolumeDriver::$mimetypes)
support Data URI scheme uploading

ex. d&d HTML
'<img src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAUA
AAAFCAYAAACNbyblAAAAHElEQVQI12P4//8/w38GIAXDIBKE0DHxgljNBAAO
9TXL0Y4OHwAAAABJRU5ErkJggg==">'