Add filestream to an existing table column

I have the following table column:

[Content] [varbinary](max) NULL

And I want to make it a filestream column so I tried:

alter table dbo.Files
  alter column Content add filestream

But I get the error:

Incorrect syntax near 'filestream'.  

I also tried

alter table dbo.Files
  alter column Content varbinary(max) filestream not null

But I got the error:

Cannot alter column 'Content' in table 'Files' to add or remove the FILESTREAM column attribute.

How can I add filestream to an existing column?