Sorry... not possible with the current implementation of the Flie Listener.
input-suffix with an empty string has worked in my tests (JBoss AS 4.2.3 & ESB 4.5) for files without extensions. But configuring it that way will consume files with an extension too (so you shouldn't use the same input directory).
In the Programmer's guide about the FTP Listener configuration (page 86) you can read :
The file suffix used to filter files targeted for
comsumption by the ESB (note: add the dot,
so something like '.esbIn'). This can also be
specified as an empty string to specify that
all files should be retrieved.