  Context: GRB afterglows are excellent probes of gas and dust in star-forming
galaxies at all epochs. It has been posited that dust in the early Universe
must be different from dust at lower z. To date two reports directly support
this contention, one of which is based on the spectral shape of GRB 050904 at z
= 6.295. Aims: We reinvestigate the afterglow to understand dust at high z. We
address the claimed evidence for unusual (SN-origin) dust in its host galaxy by
simultaneously examining the X-ray and optical/NIR spectrophotometric data.
Methods: We derive the intrinsic SED of the afterglow at 0.47, 1.25 and 3.4
days, by re-reducing the Swift X-ray data, the 1.25 days FORS2 z-Gunn
photometric data, the spectroscopic and z'-band photometric data at ~3 days
from the Subaru telescope, as well as the critical UKIRT Z-band photometry at
0.47 days, upon which the claim of dust detection largely relies. Results: We
find no evidence of dust extinction in the SED. We compute flux densities at
lambda_rest = 1250 AA directly from the observed counts at all epochs. In the
earliest epoch, 0.47 days, the Z-band suppression is found to be smaller (0.3
+- 0.2 mag) than previously reported and statistically insignificant (<1.5
sigma). Furthermore we find that the photometry of this band is unstable and
difficult to calibrate. Conclusions: From the afterglow SED we demonstrate that
there is no evidence for dust extinction -- the SED at all times can be
reproduced without dust, and at 1.25 days in particular, significant extinction
can be excluded, with A(3000 AA) < 0.27 mag at 95% confidence using the SN-type
extinction curve. We conclude that there is no evidence of any extinction in
the afterglow of GRB 050904 and that the presence of SN-origin dust in the host
of GRB 050904 must be viewed skeptically. [abridged]
