Try Reconstructing a file from a packet dump and here are a few snippets I collected here, written by others, which may prove useful. Untested. :-)

UPDATE: I just tested them, and these don't work, so I delete them, and replace them with this:

# supppose you have raw dumps , created with something like:
tcpdump -i eth0 -w tcpdump.out # to capture raw 
# then do this:
tcpdump -r ./tcpdump.out > tcpdump.outr

the ouptut will look then like:
....
....
12:17:28.864666 IP oreilly.com.http > zenlap.zentara.net.22322: . ack 
+651 win 111 <nop,nop,timestamp 1111732560 25
12:17:29.062812 IP oreilly.com.http > zenlap.zentara.net.22322: . 1:14
+41(1440) ack 651 win 111 <nop,nop,timestamp 
.....
.....
[download]

Is that what you want?