MimeMagic: Don't seek before BOF

This has weird side effects like only extracting the tail of the
file partially or not at all.

Cherry-picked from d96c6f01479ef

Bug: 66428
Change-Id: I182128c6958244f1515227ee742c3206a7484aee
Merge "MimeMagic: Don't seek before BOF" into REL1_23