If you add Products.OpenXml to your instance eggs and install it in Plone you can index modern Office formats, at least .docx and .xlsx. For plain old Excel (.xls) files this does not work.
I tried it in a Plone 4.3.2 buildout config a few weeks ago:
[instance]
eggs =
...
Products.OpenXml
[versions]
# You need a more recent lxml than default Plone, some 3.x version
lxml = 3.3.3
Products.OpenXml = 1.1.1
Alternatively or additionally, use Products.AROfficeTransforms. I have only tried it in combination with Products.OpenXml, but Products.AROfficeTransforms on its own is sufficient if you are only interested in old-style excel sheets, .xls. In a buildout config:
[instance]
eggs =
...
Products.AROfficeTransforms
[versions]
Products.AROfficeTransforms = 0.11.0
It requires the xlhtml binary to be installed on your system. This is an ancient binary, last changed in 2002. I did not try to install it myself.