Data-carrying header lines, like X-Face

I apologize that this is only tangetially related to procmail...

Where can I find a decoder for X-Face: header lines?  I assume
the image of the sender's face is stored there...

On a related note, is there a limit to how long a header line
can be?  If there is no hard limit, what is the recommended
length to stay within?  I would imagine if one exceeds 1024
(the typical shell line buffer size) there will be trouble
somewhere...

Paul