  We present the results of a new study of the relationship between infrared
excess (IRX), UV spectral slope (beta) and stellar mass at redshifts 2<z<3,
based on a deep Atacama Large Millimeter Array (ALMA) 1.3-mm continuum mosaic
of the Hubble Ultra Deep Field (HUDF). Excluding the most heavily-obscured
sources, we use a stacking analysis to show that z~2.5 star-forming galaxies in
the mass range 9.25 <= log(M/Msun) <= 10.75 are fully consistent with the
IRX-beta relation expected for a relatively grey attenuation curve, similar to
the commonly adopted Calzetti law. Based on a large, mass complete, sample of 2
<= z <= 3 star-forming galaxies drawn from multiple surveys, we proceed to
derive a new empirical relationship between beta and stellar mass, making it
possible to predict UV attenuation (A_1600) and IRX as a function of stellar
mass, for any assumed attenuation law. Once again, we find that z~2.5
star-forming galaxies follow A_1600-mass and IRX-mass relations consistent with
a relatively grey attenuation law, and find no compelling evidence that
star-forming galaxies at this epoch follow a reddening law as steep as the
Small Magellanic Cloud (SMC) extinction curve. In fact, we use a simple
simulation to demonstrate that previous determinations of the IRX-beta relation
may have been biased toward low values of IRX at red values of beta, mimicking
the signature expected for an SMC-like dust law. We show that this provides a
plausible mechanism for reconciling apparently contradictory results in the
literature and that, based on typical measurement uncertainties, stellar mass
provides a cleaner prediction of UV attenuation than beta. Although the
situation at lower stellar masses remains uncertain, we conclude that for 2<z<3
star-forming galaxies with log(M/Msun) >= 9.75, both the IRX-beta and IRX-mass
relations are well described by a Calzetti-like attenuation law.
