How much data do I leak if I use the first bytes of a file as AES-CBC IV?
The danger I see with using the beginning of a file as the IV is that the beginning of files tend to be fairly predictable.  For example. Linux shell scripts tend generally have a first line of "#!/bin/bash" - 11 characters (12 counting the Linux end-of-line LF character). If each file has a separate key this isn't much of a risk, but if the same key is used for multiple files then the IV reuse can result in problems (see this question or this question, or this question for examples)