Hello @Yannis Chaviaras (Admin) ,
Welcome to Q&A Forum!
As the article said, the filetype .potx is not included in the SharePoint Online File Type List and hence potx files are not parsed by the SharePoint search crawler.
Based on my research, we could not add or delete a file type from the search index in SharePoint Online (which can be implemented in SharePoint 2013/16/19), it is by design.
For more information, please refer to Add or remove a file type from the search index in SharePoint Server.
Thanks,
Echo Du
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