A regular expression that must be matched to accept the file example A-Za-z* (filename can only have a-z letters)
For example, if you want to require that uploads must have a wordprocessing file extension, you could enter .*.(odt|sxw|doc|dot|rtf|wpt|frm|wpd|txt|ODT|SXW|DOC|DOT|RTF|WPT|FRM|WPD|TXT)