securityonion-bro-scripts: update extract.bro for Bro 2.4

[dougburks]

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

"File analysis
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.
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)."

[dougburks]

submitted for testing:
https://groups.google.com/d/topic/security-onion-testing/g7GxUYk5T3c/discussion

[dougburks]

updated script:
https://github.com/Security-Onion-Solutions/securityonion-bro-scripts/blob/master/file-extraction/extract.bro

[dougburks]

Published:
http://blog.securityonion.net/2015/08/bro-24-now-available-for-security-onion.html