Its actually coming in from an http POST.

I think the issue is not necessarily the format that the "file" is in,
more the fact that it is in memory rather than on disk. I've thought
about what you said and I think even if the file was in csv format,
say, I'd still need either to parse it myself (yuk), or to get it onto
disk for Excel to parse (which is the same problem that I already
have!). Plus, either of these options involves saying to the users "you
must export the file in such-and-such a format for this to work", which
is a lot more hassle than saying to them "just save it as a regular xls
file".

I've bitten the bullet for now and saved the file temporarily on the
hdd, but I'm going to keep thinking about this one since hitting the
hdd will slow things down somewhat.

Any further ideas appreciated.