Filezilla FTP Server can give a cryptic “filename invalid” message. Example:

(000004)9/1/2010 2:17:25 AM - backups (X.X.X.X)> STOR /var/lib/mysql/test/items.MYD
(000004)9/1/2010 2:17:26 AM - backups (X.X.X.X)> 550 Filename invalid

Or, vsFTPD can give a similar cryptic message “could not create file” message. Example:

Sep 11 07:22:23 unknown vsftpd[8035]: [backups] FTP command: Client "X.X.X.X", "STOR ./var/lib/mysql/test/items.MYD"
Sep 11 07:22:23 unknown vsftpd[8035]: [backups] FTP response: Client "X.X.X.X", "553 Could not create file."

It means that the filename specified (on upload) cannot be stored on the server because of invalid characters. The most common culprit is when you are trying to use the manual ftp command line, and calling
put /var/local/mysql/test/items.MYD

To fix this, you must change the local directory on upload
lcd /var/local/mysql/test/
and then call
put items.MYD