Show warning if slash is entered as filename

Signed-off-by: Morris Jobke <hey@morrisjobke.de>

apps/files/js/files.js

@@ -128,6 +128,8 @@
 				throw t('files', '"{name}" is an invalid file name.', {name: name});
 			} else if (trimmedName.length === 0) {
 				throw t('files', 'File name cannot be empty.');
+			} else if (trimmedName.indexOf('/') !== -1) {
+				throw t('files', '"/" is not allowed inside a file name.');
 			} else if (OC.fileIsBlacklisted(trimmedName)) {
 				throw t('files', '"{name}" is not an allowed filetype', {name: name});
 			}