`--filter-files` does not consider extensions

[AbdealiLoKo]

To reproduce:

$ mkdir -p folder
$ echo -e 'import unknown\nimport sys' > folder/file.py
$ isort folder --check -v
ERROR: folder/file.py Imports are incorrectly sorted and/or formatted.

Try filtering files with --gitignore and --filter-files - works fine.

$ echo '*.py' > folder/.gitignore
$ isort folder --check --gitignore
Skipped 1 files
$ isort folder/file.py --check --gitignore
ERROR: folder/file.py Imports are incorrectly sorted and/or formatted.
$ isort folder/file.py --check --gitignore --filter-files
Skipped 1 files

Try filtering files with --ext and --filter-files - works fine.

$ isort folder --check
ERROR: folder/file.py Imports are incorrectly sorted and/or formatted.
$ isort folder/file.py --check --ext txt
ERROR: folder/file.py Imports are incorrectly sorted and/or formatted.
$ isort folder/file.py --check --ext txt --filter-files
ERROR: folder/file.py Imports are incorrectly sorted and/or formatted.

I expected isort folder/file.py --check --ext txt --filter-files to have no filtered out the folder/file.py