There are a few ways.
ns_http is indirect.
By examining the code associated with the url where one would click to see text file contents from file-storage ie file-storage/www/download/index.vuh the file-storage way is with: cr_write_content
The api-doc[1] states that adding "-string" to parameters will return content as string instead of sending to connection.
1. https://openacs.org/api-doc/proc-view?proc=cr_write_content&source_p=1
cheers,
Ben