Map empty mimetypes to octet-stream for Oracle DBMS.

lib/private/files/cache/cache.php

@@ -64,6 +64,10 @@ class Cache {
 	 * @return int
 	 */
 	public function getMimetypeId($mime) {
+		if (empty($mime)) {
+			// Can not insert empty string into Oracle NOT NULL column.
+			$mime = 'application/octet-stream';
+		}
 		if (empty(self::$mimetypeIds)) {
 			$this->loadMimetypes();
 		}