Save Feynman diagram files in a subfolder

Question

Suppose one has a simple Feynman diagram, such as the following MWE (which has to be run with -shell-escape, I guess this is to allow compilation to "escape the shell" to run metapost).

\documentclass{article}
\usepackage{amsmath,amssymb,braket,feynmp-auto}
\newenvironment{feynman}[1]{\begin{center}\begin{fmffile}{#1}\vspace*{\baselineskip}}{\vspace*{\baselineskip}\end{fmffile}\end{center}}
\begin{document}
\begin{feynman}{01}
    \begin{fmfgraph*}(110,65)
        \fmfleftn{i}{2}
        \fmfrightn{o}{1}
        \fmf{fermion}{i1,v1,i2}
        \fmf{dashes,label=$\braket{\lambda_2|T|\lambda_1}$}{v1,o1}
        \fmfdotn{v}{1}
        \fmfv{decoration.shape=cross,decoration.filled=empty,decoration.size=0.1w}{o1}
        \fmflabel{$\lambda_2$}{i1}
        \fmflabel{$\lambda_1$}{i2}
    \end{fmfgraph*}
\end{feynman}
\end{document}

In the case of a large project, one wouldn't like to have all the .1, .log, .mp and .t1 files to be in the main folder, but rather in a subfolder. When the name of the diagram is changed to for example diagrams/01, by changing the 5th line to

\begin{feynman}{diagrams/01}

only the file 01.mp appears in the subfolder diagrams, the other files aren't created (it seems the second step in creating the diagram doesn't recognize the path), with as result no output in the pdf.

Question: How can we get whatever script/program that is being run to recognize the path to the .mp file? Or, more generally, how can we get the diagram files to be stored in a subfolder?