Add initial stab at mime_type for file objects

[dcode]

Address #749 file objects.

While "Content-Type" in HTTP headers and related protocols utilize
MIME type descriptions, they are offered as a hint to the other
end of the connection so they know how to handle the data. However,
in the context of a file, the MIME type should be more authoritative
in that tools like Zeek, Suricata, and other tools that use
libmagic or yara to detect filetypes provide information based upon
a match to a pattern of bytes.

For #554, I think retaining the headers as Content-Type makes sense
as this is the configured value set by the server (or client). I think
it is useful to compare what was advertised vs what was actually sent,
e.g. the header value in http.response.content-type vs file.mime_type
of the file transferred.

[webmat]

Thanks @dcode!

I agree with adding it only to file, to start. Only one small adjustment to the description, and this can go in 👍

In the future, please make the reformatting into its own commit, then the actual changes as different commits. Makes the reviewing easier :-)

[webmat]

Ping @neu5ron

[webmat]

LGTM, thanks for the adjustment.

I'll take care of the changelog conflict when merging.