fix(fs): make filter_files_by_extension return only files

[DeckerSU]

By default, get_wallet_names returns not only *.dat filenames from db_root as wallet names but also directories. For example, if we create a l.dat folder in db_root, it will be included in the wallet names list, like this:

{
  "mmrpc": "2.0",
  "result": {
    "wallet_names": [
      "l",
      "decker",
      "hello"
    ],
    "activated_wallet": "decker"
  },
  "id": null
}

The current implementation is not necessarily ideal. Maybe we should perform this check (i.e., ensuring it’s a file and not a directory) directly in filter_files_by_extension. But my primary goal was to avoid modifying existing file-related functions.

[onur-ozkan]

Looks good to me

[shamardy]

Thanks for the find and the fix!