Overleaf can't find upright font files in folders

Question

Overleaf renders a pdf properly when the upright font file is in the root folder, but returns an error when the file is in a folder.

Here's the source

\documentclass{article}
\usepackage{fontspec}
\setmainfont[
  ItalicFont={fonts/LinLibertine_RIah.ttf},
  ]{LinLibertine_Rah.ttf}

\begin{document}

Hello \emph{world}

\end{document}

and here's the file structure.

but if I move the upright file into the same folder, changing the source thus

\documentclass{article}
\usepackage{fontspec}
\setmainfont[
  ItalicFont={fonts/LinLibertine_RIah.ttf},
  ]{fonts/LinLibertine_Rah.ttf}

\begin{document}

Hello \emph{world}

\end{document}

and updating the file structure

returns an error.

How do I move properly refer to the upright font file when its in a folder?

Answer 1

[Disclaimer: I'm a support personnel at Overleaf]

Actually your code snippet doesn't work either when I tried compiling it on a local TeX Live installation. But using the Path option does:

\setmainfont[Path=fonts/,
             ItalicFont=LinLibertine_RIah.ttf]
  {LinLibertine_Rah.ttf}