import pandas as pd
import os

def allowed_file(filename):
    return '.' in filename and filename.rsplit('.', 1)[1].lower() in {'csv', 'xls', 'xlsx'}

def read_file(filepath):
    if filepath.endswith('.csv'):
        return pd.read_csv(filepath)
    else:
        return pd.read_excel(filepath)

def save_processed_file(processed_df, filepath):
    processed_df.to_csv(os.path.join(os.path.dirname(filepath), 'processed_' + os.path.basename(filepath)))
    return 'processed_' + os.path.basename(filepath)