How would you validate the extension of file which could be applicable for all Appian objects or entire application?

In Appian, the primary way to validate file extensions across an entire application is by using the file upload in the Administration Console

Navigate to the Admin Console: Access the "Appian Administration Console" from the settings menu.
Configure File Upload Settings: Within the console, you can manage "File Upload" settings to define which file extensions are permitted or blocked.
Enable File Type Verification: Toggle on the checkbox labeled "Block any file with an extension that does not match the underlying file type".

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