On occasions - you may experience a "corrupt file' message when uploading the Emailer Module to WHMCS billing system. This may be due to the file being encrypted as the Filezilla client considers .php files for ASCII mode FTP transfer.
Open Filezilla FTP client, go to the Transfer > Transfer type > Binary
Changing this mode to binary ensured that the binary files do not get corrupt while upload or download. You can permanently set this mode to Binary because Binary Transfer Mode supports both binary as well as ASCII files.