With express.js how do I configure an upload file directory only for testing?

I have a form with a file upload field. I want to upload the file during a test. Normally the file is placed in the static file folder in the files folder. In production the static folder is build and in development the static folder is client however I want the uploaded files to be saved and served from tmp/ during testing or, at least, in a directory I can delete after each test. Right now the test and development configurations use the same directory. Is there an elegant solution to this?