
This function tries to ensure what applied data have a pointed encoding mode.
If mode parameter is 0 (ie unknown) or if the validation fails,
it will try to determine which mode was used in fact.
    
RFC 1521,
(N. Borenstein, N. Freed.
MIME (Multipurpose Internet Mail Extensions) Part One:
Mechanisms for Specifying and Describing
the Format of Internet Message Bodies),
contains detailed description of most important encodings used by mail
systems.
RFC 2045,
(N. Borenstein, N. Freed.
MIME (Multipurpose Internet Mail Extensions) Part One:
The Format of Internet Message Bodies).
    
Currently, seven conventions are used for mail attachments.
In Virtuoso, they are enumerated by integer IDs.
    