```python
import pandas as pd

class Files:

    def __init__(self, db='files.sqlite'):
        import sqlite3
        if not db.endswith('.sqlite'): db += '.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)
```