File types restrictions
The editor accepts multiple files at a time and limits the extensions to some predefined, common file types.
You can change these settings in
Options -> Security. Any file type can be added to the allowed extensions list. Even .php files, but you will get a warning indicating the security risk in doing so.
Another important setting allows you to select who is limited by these extension list. You can enable the restriction for all users and clients, clients only or no one.