Forms >>> Check Entry.
Prevents the user from selecting a filename with a space (known to cause problems with some CGI-scripts). The visitor are informed that the field can not have spaces. Of course, you could change the character it checks for as well as the invalid alert message to something else to suit your needs.Try to select a file with a space in it's name or in it's path!
Add the below code to the <body> section of your page: