info ls, section 10.1.2, What information is listed (from the manual for GNU coreutils 8.5):
Following the file mode bits is a single character that specifies
whether an alternate access method such as an access control list
applies to the file. When the character following the file mode
bits is a space, there is no alternate access method. When it is
a printing character, then there is such a method.
@ indicates that there exists an alternate access method on the file. This most likely means an ACL. Try
getfacl Dropbox and see what you get.
You may need to install some package to get the ACL-related commands. In Debian,
getfacl is provided by the
acl package, so
sudo apt-get install acl if it isn't installed already. Other distributions may or may not use the same package name.