You need to look further down in the documentation, where it says

The $filename is the filename to report in the request. If this value is undefined, then the basename of the $file will be used.

so no, I don't think that's what the problem is. I would start by making sure that "/maintenance/instructions" is in fact a script that expects a file to be posted from a form, and also take a look at what output that script is producing when you call it in this way.