  We perform the first numerical simulations modeling the inflow and outflow of
magnetized plasma in the Kerr-Sen spacetime, which describes classical spinning
black holes (BHs) in string theory. We find that the Blandford-Znajek (BZ)
mechanism, which is believed to power astrophysical relativistic outflows or
``jets'', is valid even for BHs in an alternate theory of gravity, including
near the extremal limit. The BZ mechanism releases outward
Poynting-flux-dominated plasma as frame-dragging forces magnetic field lines to
twist. However, for nonspinning BHs, where the frame-dragging is absent, we
find an alternate powering mechanism through the release of gravitational
potential energy during accretion. Outflows from non-spinning stringy BHs can
be approximately $250\%$ more powerful as compared to Schwarzschild BHs, due to
their relatively smaller event horizon sizes and, thus, higher curvatures.
Finally, by constructing the first synthetic images of near-extremal non-Kerr
BHs from time-dependent simulations, we find that these can be ruled out by
horizon-scale interferometric images of accreting supermassive BHs.
