Extension to add to images and attachments when the dtsExoLimitExtensions flag is set and an image or attachment has an extension that is not listed in AllowedExtensions.
The AllowedExtensions list controls which attachments and images can be extracted with the original filename extension preserved. Attachments and images with extensions that are not in the list will have the DefaultExtension appended to the output filename, effectively disabling the original extension.
Do not include the "." in the extension.