> Rename the file to have a .ttf extension; it is and OpenType font with
> TrueType outlines and those usually have .ttf extension (though the
> spec says they can have .otf extension, ConTeXt does not handle this
> right now).
>
> Regards,
> Khaled

Thanks Khaled, now it works, at least to some degree. I have still 
problems, which I dont have with MS Word, see this example:

\definefont[amiri][file:UthmanTN1Ver10.ttf*arabic at 17 pt]
\define[1]\Arab{\setupalign[r2l]\amiri#1}
\starttext
\Arab{
نَسْتَعِينُ وَلَا الضَّالِّينَ
}
\stoptext

The vocalisations are set wrong, see attachment. Specially with a final 
"nuun" there are problems. It is not a problem of the font, because in 
Word they are set correctly. Does this come from the extensions ConTeXt 
do not support, or is there an easy way to fix?
Or asked more generally, if fixing is not possible: Do you know any 
font, which can correct handled by context, and is capable of 
typesetting quran?

Thanks.
Huseyin