class file_mime_body :?fin:bool -> string ->mime_body
An implementation of mime_body where the value is stored
in an external file.
The argument is the name of the file containing the (decoded) value.
The method store returns `File filename.
The method value loads the contents of the file and returns them
as string.
Example: To create a body from the file "f", call
new file_mime_body "f" fin : whether to delete the file when the finalize method is called
(default: false)