/var/opt/open-xchange/filestore or somesuch, IIRC. You can look that up in the configdb in the table 'filestore'. Below that directory is a directory for the context and below that a three structure simply with identifiers (00/00/00, 00/00/01, 00/00/02... 00/01/01 ... ). The filenames you see in the infostore come exclusively from the database.
Unless backing up (or clustering) your system, you usually wouldn't have to bother with the physical location of the file-data.
Why do you ask? What are you trying to accomplish?