Empty mime type in http.log


I’m trying to use Bro to identify content types in http traces - so I’m looking at the orig_mime_types and resp_mime_types columns in http.log. Now, I captured a trace where I blocked all applications except Google chrome, and then used Chrome to play a bunch of videos on Dailymotion. Then I ran Bro over this trace and then ran the following command over the output:

cat http.log | bro-cut orig_mime_types resp_mime_types | sort | uniq -c

This is the output I got:

88 - -
35 - application/octet-stream
12 - application/xml
25 - image/gif
188 - image/jpeg
1 - image/png
13 - text/html
59 - text/plain
24 text/plain text/plain

I can’t figure out why blanks are appearing in so many of the lines. There are 88 lines where the content type (in both resp and orig columns) is empty - what does that signify? I did see that Bro uses signature matching to correct missing or incorrect file types. Does a “-” mean Bro wasn’t able to match any signature to the content being transferred? Or that it found nothing in the HTTP header’s content type field?