[Bro] Converting fa_file to file_sniff

[James Lay]

So per:

https://www.bro.org/download/NEWS.bro.html

"Removed fa_file record’s mime_type and mime_types fields. The event 
file_sniff has been added which provides the same information. The 
mime_type field of Files::Info also still has this info."

I have a script...smtp-file-extract.bro:

global ext_map: table[string] of string = {
         ["application/x-dosexec"] = "exe",
         ["application/zip"] = "zip",
         ["application/msword"] = "xls",
};

event file_new(f: fa_file)
         {
         if ( f$source != "SMTP" )
                 return;

         if ( ! f?$mime_type || f$mime_type !in ext_map )
                 return;

         local ext = "";

         if ( f?$mime_type )
                 ext = ext_map[f$mime_type];

         local fname = fmt("%s-%s.%s", f$source, f$id, ext);
         Files::add_analyzer(f, Files::ANALYZER_EXTRACT, 
[$extract_filename=fname]);
}

which while not perfects gets what I need done.  This is now broken with 
2.4, as expected, however I'm at a loss on how to fix this.  Again, per 
the NEWS link above:

"The earliest point that new mime type information is available is in 
the file_sniff event which comes after the file_new and 
file_over_new_connection events. Scripts which inspected mime type info 
within those events will need to be adapted. (Note: for users that 
worked w/ versions of Bro from git, for a while there was also an event 
called file_mime_type which is now replaced with the file_sniff event)."

Awesome.  How do I adapt this?  Not sure where to look for changing 
this.  Thank you.

James