[XeTeX] ".ttf" automatically added to font names

[Dirk Hünniger]

Hello,
my name is Dirk Hünniger. Since approx 10 years I am working on 
mediawiki2latex a pdf and latex export tool for mediawiki. I am 
currently using Ubuntu 17.04 and observe a behavior of the \setmainfont 
command that I don't understand. The string ".ttf" seems to be added 
magically, which causes problems for me.

Everything works fine if I write it like this:

\setmainfont[Path=/usr/share/fonts/truetype/cmu/,UprightFont=cmunrm,BoldFont=cmunbx,ItalicFont=cmunti,BoldItalicFont=cmunbi]{cmunti}

I am using

This is XeTeX, Version 3.14159265-2.6-0.99996 (TeX Live 2016/Debian) 
(preloaded format=xelatex)
  restricted \write18 enabled.

In an older version of xetex I had to write it like this.

\setmainfont[Path=/usr/share/fonts/truetype/cmu/,UprightFont=cmunrm.ttf,BoldFont=cmunbx.ttf,ItalicFont=cmunti.ttf,BoldItalicFont=cmunbi.ttf]{cmunrm.ttf}

If I do this now I get the following error:

> kpathsea:make_tex: Invalid fontname 
> `[/usr/share/fonts/truetype/cmu/cmunrm.ttf.ttf]/OT', contains '['
> kpathsea:make_tex: Invalid fontname 
> `[/usr/share/fonts/truetype/cmu/cmuntt.ttf.ttf]/OT', contains '['
>
> !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
> !
> ! fontspec error: "font-not-found"
> !
> ! The font "cmuntt.ttf" cannot be found.
> !
> ! See the fontspec documentation for further information.
> !
> ! For immediate help type H <return>.
> !...............................................
>
> l.24 ...ttf,BoldItalicFont=cmuntx.ttf]{cmunbx.ttf}
>

So the string .ttf is automatically added which causes the problem. The 
problem gets serious If I want to uses east-asian fonts like

/usr/share/fonts/truetype/wqy/wqy-zenhei.ttc

which does not end on ttf. So I propose to revert this change so that 
the file extension .ttf is not automatically added and thus has to be 
written out by the user of xetex.
Yours Dirk Hünniger