So there are a could approaches to this. On windows 7 at lease chrome defaults to the users download directory so something like this: C:\Users[username]\Downloads. So you could just code that paths into your test. Another option would be to create a specific test profile that specifies a specific path and you can specify it in the options to use that profile:
ChromeOptions options = new ChromeOptions();
options.addArguments("user-data-dir=/path/to/your/custom/profile");
A third option would be to skip all the file IO stuff and use a web proxy tool to intercept the package and just deal with the data in code. This is an approach I have taken before though in c#.