  Fundamental properties of the extinction curve, like the slope in the
rest-frame ultraviolet (UV) and the presence/absence of a broad absorption
excess centred at 2175 A (the UV bump), are investigated for a sample of 108
massive, star-forming galaxies at 1 < z < 2.5, selected from the FDF
Spectroscopic Survey, the K20 survey, and the GDDS. These characteristics are
constrained from a parametric description of the UV spectral energy
distribution (SED) of a galaxy, as enforced by combined stellar population and
radiative transfer models for different geometries, dust/stars configurations
and dust properties. In at least one third of the sample, there is a robust
evidence for extinction curves with at least a moderate UV bump. The presence
of the carriers of the UV bump is more evident in galaxies with UV SEDs
suffering from heavy reddening. We interpret these results as follows. The
sample objects possess different mixtures of dust grains and molecules
producing extinction curves in between the average ones of the Small and Large
Magellanic Cloud, where the UV bump is absent or modest, respectively. Most of
the dust embeds the UV-emitting stellar populations or is distributed out of
the galaxy mid-plane. Alternatively, even dust with a pronounced UV bump, as
for the average Milky-Way extinction curve, can be present and distributed in
the galaxy mid-plane. In this case, variations of the continuum scattering
albedo with wavelength or an age-dependent extinction are not sufficient to
explain the previous trend with reddening. Hence, additional extraplanar dust
has to be invoked. The data suggest that the carriers of the UV bump are
associated with intermediate-age stellar populations, while they survive in the
harshest UV-radiation fields owing to dust self-shielding. [abridged]
