`--filter-files` does not consider extensions

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

Start with the isort CLI handling for --filter-files, --ext, and --gitignore, then run the reproduction commands from the issue to compare their filtering behavior. Done means a file whose extension is excluded by --ext is skipped when --filter-files is used, while the existing gitignore behavior remains unchanged.