Hi Ram,
As said by other experts, the originals (Attachments) can be uploaded in a DIR using either 'Originals' section or through 'Originals' tab. If these section/tab is not visible, then check the configuration of a document type (tcode-DC10) for suppressed fields((Screenshot attached).
Document type configuration screenshot:
And to answer your second question, 'Additional data' tab appears in DIR only if it is configured in the document type(shown in the above screenshot) or if you are manually linking the class to a DIR using classification button(Appear above the document number).
In configuration, only one class can be linked and if multiple classes are to be linked then manual method can be used.
Hope this helps.
Regards,
Ravi