Which SQL data type to store Base64 encoded file?
Well, Base64 is a ASCII encoding, really - so definitely no need for NVARCHAR - and since it's text, I'd suggest VARCHAR(MAX)
It's pure text, up to 2 GB (should be enough), and it's a string-type, so you can use all string functions on it. NVARCHAR does indeed use twice as much storage - always 2 bytes per character - and is totally unnecessary in this case.