The modern spiffs backend doesn't like stat("./somefile") for something
on the root of the filesystem, and instead only accepts "somefile"
(it also doesn't like "/somefile"). The error from stat was being
ignored which is why the file sizes all appeared to be zero.
The fix is to change file.list() to pass just the filename, unless a
directory was passed to list(). Also improved error handling a bit.
e3883cd27b