An extension filter extracts a file attachment extension using an email's MIME headers and applies a decision based on the extension filter settings.

Go to Filtering > Attachments and select the Extension Filters tab. Extension filters that have been created at a higher level of access and applied to your domain(s) are displayed with "(inherited)" following the filter name – for example, js (inherited).


If a filename extension has been modified, it may not be recognized by an extension filter. For example, if an executable file, photo.exe, has been renamed to photo.jpg, a .exe extension filter will not detect this change. In this situation, a File Type Filter and/or a MIME Type filter are also necessary.

Creating an Extension Filter

  1. Go to Filtering > Attachments and select the Extension Filters tab.

  2. Select Create and the Create An Extension Filter window displays.

    • Extension: Enter a file extension. Only alphanumeric characters are allowed for filename extensions.

    • Inbound action: Select the action to take on inbound emails that match this filter:

      • Allow: Allow the email.

      • Block: Block the email.


        The domain or user policy determines if the message is quarantined. Go to Policies > Domain Policy or User Policies.

      • Ignore: Take no action.

    • Scan Archives: Select if you want this filter to match files contained within compressed archive files.

    • Scan Double Extensions: Select to identify attachments that have been renamed to conceal their true type, as double extensions can be used to trick a user into opening malware. A mail client may hide a second extension, so filename.gif.exe appears as filename.gif.

    • Comments (optional): Enter optional comments about this extension filter.

  3. Select Create or save this extension filter or Cancel to discard changes.

This article relates to SpamTitan Skellig.