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
