I think this is a reasonable approach, as long as (per Alex's
suggestion) it's configurable in various ways.
E.g. if you know you don't want to parse OLE2-based files, so you've
removed jars for those parser, then it would be great to have an easy
way of disabling the (more expensive) mime-type detection, and
potentially avoid the dependency on these same jars.
Separately, I think this issue might also trigger improvements to the
existing "magic bytes" detection code in Tika. IIRC, we wound up
adding full regex with some additional matching rules in Krugle, to
extend the (from Nutch, same as Tika) mime-type detection code to
better handle things like source code files. I imagine something
similar might be needed to reliably handle container matching.
-- Ken
On Jun 15, 2010, at 10:25am, Nick Burch wrote:
Hi All
I've been thinking about TIKA-391 (intermittent incorrect mime type
detection of office formats), and I think we might need to do
something different for container formats.
At the moment, for OLE2 based files (.xls, .ppt, .doc, .msg, .vsd
etc), and for ZIP based files (.zip, but
also .xlsx, .pptx, .docx, .odf, .odt, .ots, .sxw etc), I don't think
the current method works well. AFAICT,
we detect the container, then have sub-class matches that try to
look for the appropriate children by hoping we can guess where the
definition might hide within the container. However, I think this is
too unreliable - for example, with a .doc file, the entry for the
Word stream can come anywhere in the list of top level entries, so
is very hard to reliably find without properly parsing the OLE2
structure
So, I'd like to suggest a slightly different approach, one of
loading the container format to decide the mime type. This will, of
course, make the detection step slower and more memory hungry for
detecting these (but only these) kinds of documents. However,
provided that we keep the open container around and pass it to the
parser in a later step, it's work we would've done anyway.
I'd then see the mime process be something like:
* Loop over all magic rules
* If the magic fits and the file extension fits, pick this one
* Otherwise if the magic fits and it's a container:
* Load the container
* Check the top level entries against our list for that container
* If we get a hit, pick that
* If nothing hits, assume it's just the container
eg we have a file with the zip magic, but no / unreliable filename.
We open the zip file and look at the top level directory entries.
If we spot [Content_Types].xml and /xl/ we know it's an OOXML Excel
file
If we spot meta.xml and mimetype then read mimetype and go from there
...
Else decide it's just a zipfile of files, and handle appropriately
What does everyone else think? Is the extra work in the mime
detection step (but only for container formats with no reliable
filename) worth it for the improved detection?
note - the issue of when given a filename with a useful extension of
being
able to reliably pick the right mime type still needs to be solved,
but
largely wouldn't be affected by this
Nick
--------------------------------------------
Ken Krugler
+1 530-210-6378
http://bixolabs.com
e l a s t i c w e b m i n i n g
