If attachments are showing up in your Microsoft Outlook inbox with a modified ending such as .doc being changed to .1324DEFANGED-doc then the issue is more than likely with your mail server instead of your email client. There is an application called MailScanner that modifies the file ending as a security measure to stop any files from being executed automatically. Below I briefly discuss modifying this setting on a Linux server using ISPConfig.
The easiest way to resolve this issue on a per user basis with an ISPConfig Linux server is to disable MailScanner for that user. MailScanner does provide numerous other security checks though so it is not recommended to disable it but if you must disable the file ending modification then modify the below files..doc, attachments, DEFANGED, extensions, ISPConfig, Linux, mailscanner, MS Word, Outlook