I don't think it is a good idea to make the default content-type
depend on the URL scheme. Is it 'text/plain; charset=US-ASCII' for
'mailto:', but 'text/plain; charset=ISO-8859-1' for 'http:', but what
is it for ftp:? Or some new URL scheme? It just isn't that much extra
overhead to include the actual content-type if it isn't the MIME
default. If you want to optimize form data encapsulation to save
space, compress the data stream. Similarly for the
content-transfer-encoding field. This is a small fixed overhead for
each part.
In addition, I don't think RFC 1522 encoding is appropriate to
introduce into multipart/form-data headers; it's a complex mechanism
for little or no benefit. The field anmes are not themselves visible
to end users of the application.