  Magnetar giant flares (MGFs) have been long proposed to contribute at least a
sub-sample of the observed short gamma-ray bursts (GRBs). The recent discovery
of the short GRB 200415A in the nearby galaxy NGC 253 established a
textbook-version connection between these two phenomena. Unlike previous
observations of the Galactic MGFs, the unsaturated instrument spectra of GRB
200415A provide for the first time an opportunity to test the theoretical
models with the observed $\gamma$-ray photons. This paper proposed a new
readily fit-able model for the MGFs, which invokes an expanding fireball
Comptonized by the relativistic magnetar wind at photosphere radius. In this
model, a large amount of energy is released from the magnetar crust due to the
magnetic reconnection or the starquakes of the star surface and is injected
into confined field lines, forming a trapped fireball bubble. After breaking
through the shackles and expanding to the photospheric radius, the thermal
photons of the fireball are eventually Comptonized by the relativistic
$e^{\pm}$ pairs in the magnetar wind region, which produces additional
higher-energy gamma-ray emission. The model predicts a modified thermal-like
spectrum characterized by a low-energy component in the Rayleigh-Jeans regime,
a smooth component affected by coherent Compton scattering (CC) in the
intermediate energy range, and a high-energy tail due to the inverse Compton
process. By performing a Monte-Carlo fit to the observational spectra of GRB
200415A, we found that the observation of the burst is entirely consistent with
our model predictions.
