.ICHAT File Extension
What is an ICHAT file?
Save file created by iChat, an online chatting application included with Mac OS X; saves the currently opened chat log and enables the user to review past discussions; created by choosing File → Save a Copy As... in the iChat application menu.
ICHAT files are saved in a binary format and therefore cannot be viewed with a text editor. However, if you double-click an ICHAT file on a Mac, iChat will automatically open it to reveal the chat log.
NOTE: iChat was replaced by Messages in 2012.