The FileMask property specifies the default wildcard mask
to be used when uploading or downloading multiple files. The default
value of an empty string indicates that all files in the specified
directory should be uploaded or downloaded. Typically, this property
is set to a wildcard mask that limits the files downloaded from the
server to those which match a specific extension. For example, to
download only those files that end in a ".dat" extension,
the property could be set to the value "*.dat"
Note that the type of wildcards which may be used depend on the
server and the type of file system that it is using. Take particular
care when dealing with file systems that distinguish between upper-
and lower-case letters in a filename.