  The properties of dust attenuation at rest-frame UV wavelengths are inferred
from very high-quality FORS2 spectra of 78 galaxies from the GMASS survey at
1<z<2.5. These objects complement a previously investigated sample of 108
UV-luminous galaxies at similar redshifts, selected from the FDF spectroscopic
survey, the K20 survey, and the GDDS. The shape of the UV extinction curve is
constrained by a parametric description of the rest-frame UV continuum. The UV
bump is further characterised by fitting Lorentzian-like profiles. Spectra
exhibit a significant 2175A feature in at least 30% of the cases. If
attenuation is dominated by dust ejected from the galaxy main body via
superwinds, UV extinction curves in-between those of the SMC and LMC
characterise the sample galaxies. The fraction of galaxies with extinction
curves differing from the SMC one increases, if more dust resides in the
galactic plane or dust attenuation depends on stellar age. On average, the
width of the manifested UV bumps is about 60% of the values typical of the LMC
and Milky Way. This suggests the presence of dust similar to that found in the
LMC2 supershell close to 30Dor. The presence of the carriers of the UV bump at
1<z<2.5 argues for outflows from AGB stars being copious then. Consistent with
their higher SFRs, the GMASS galaxies with a manifested UV bump are more
luminous at rest-frame 8mum, where the emission is dominated by PAHs (also
products of AGB stars). In addition, they exhibit stronger UV absorption
features, mostly of interstellar origin, which indicates overall more evolved
stellar populations. We conclude that diversification of the small-size dust
component has already started in the most evolved star-forming systems at
1<z<2.5.
