The getSupportedViewExtensions method has to return a list of all file extensions (in lower case) for which the content handler shall be available as viewer application. The file extension "content" has a special meaning: If the file extension "content" is included, then the viewer is available for viewing nodes of type org.docma.plugin.PubContent (XHTML content-nodes). Otherwise the viewer is only available for nodes of type org.docma.plugin.FileContent with one of the returned file extensions.
    public String[] getSupportedViewExtensions()
        return new String[] { "content", "txt" };
In the method implementation given above, the extensions "content" and "txt" are returned. That means, the viewer is available for XHTML content-nodes and file-nodes with extension "txt".
Note: text file extensions
Docmenta allows to configure a list of text file extensions. If an extension is configured to be a text file extension, then all content handlers that support the "txt" extension are also available for this file extension. In other words, a content handler that supports the "txt" extension also supports viewing of files with an extension that has been configured as a text file extension.