Hide .crdownload files and prevent users from changing extension to it.

ui/file_manager/file_manager/common/js/util.js

Index: ui/file_manager/file_manager/common/js/util.js
diff --git a/ui/file_manager/file_manager/common/js/util.js b/ui/file_manager/file_manager/common/js/util.js
index 25af53434638b685ac5d95b681ed20b2a6b886a7..757955f6e91fdb278cf7bc7b2fba8297d8eec6ed 100644
--- a/ui/file_manager/file_manager/common/js/util.js
+++ b/ui/file_manager/file_manager/common/js/util.js
@@ -1344,6 +1344,8 @@ util.validateFileName = function(parentEntry, name, filterHiddenOn) {
     return Promise.reject(str('ERROR_WHITESPACE_NAME'));
   else if (/^(CON|PRN|AUX|NUL|COM[1-9]|LPT[1-9])$/i.test(name))
     return Promise.reject(str('ERROR_RESERVED_NAME'));
+  else if (filterHiddenOn && /\.crdownload$/i.test(name))
+    return Promise.reject(str('ERROR_RESERVED_NAME'));
   else if (filterHiddenOn && name[0] == '.')
     return Promise.reject(str('ERROR_HIDDEN_NAME'));