class file_mime_body :
?fin:bool -> string ->
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.
value loads the contents of the file and returns them
Example: To create a body from the file "f", call
new file_mime_body "f"
fin: whether to delete the file when the
finalizemethod is called (default: false)