```python
import sqlite3, pandas as pd

class Files:
    def __init__(self, db='files.sqlite'):
        self.conn = sqlite3.connect(db, isolation_level=None)
        self.conn.row_factory = sqlite3.Row
        self('PRAGMA journal_mode=WAL')
        self('CREATE TABLE IF NOT EXISTS files (filename TEXT PRIMARY KEY, content BLOB)')

    def __call__(self, *args) -> pd.DataFrame:
        return pd.DataFrame(dict(row) for row in self.conn.execute(*args))

    def __setitem__(self, filename, content) -> None:
        assert isinstance(filename, str)
        if isinstance(content, str): content = content.encode()
        assert isinstance(content, bytes)
        self('INSERT OR REPLACE INTO files VALUES (?,?)', (filename, content))

    def __getitem__(self, filename) -> bytes:
        return self('SELECT content FROM files WHERE filename=?', (filename,)).content[0]

    def __delitem__(self, filename) -> None:
        self('DELETE FROM files WHERE filename=?', (filename,))

    def __iter__(self):
        df = self('SELECT filename FROM files')
        return iter([] if df.empty else df.filename)

    def __contains__(self, filename: str) -> bool:
        return not self('SELECT 1 FROM files WHERE filename=?', (filename,)).empty

    def __len__(self) -> int:
        return self('SELECT COUNT(*) FROM files').iloc[0, 0]

files = Files()
for filename in files:
    print(files[filename])
files['hello'] = 'world'
print(files['hello'])
print(len(files))
print('hello' in files)
for filename in files:
    print(filename)

del files['hello']
for filename in files:
    print(filename)
```