如何使用Dash上传、处理和下载.csv文件？

import dash
from dash.dependencies import Input, Output, State
import dash_core_components as dcc
import dash_html_components as html
import pandas as pd
import base64

app = dash.Dash(__name__)

app.layout = html.Div([
    dcc.Upload(
        id='upload-data',
        children=html.Div([
            '拖放或点击选择.csv文件上传'
        ]),
        style={
            'width': '100%',
            'height': '60px',
            'lineHeight': '60px',
            'borderWidth': '1px',
            'borderStyle': 'dashed',
            'borderRadius': '5px',
            'textAlign': 'center',
            'margin': '10px'
        },
        multiple=False
    ),
    html.Div(id='output-data-upload'),
    html.A('下载.csv文件', id='download-link', download='', href=''),
])

@app.callback(Output('output-data-upload', 'children'),
              Output('download-link', 'href'),
              Input('upload-data', 'contents'),
              State('upload-data', 'filename'))
def update_output(contents, filename):
    if contents is not None:
        content_type, content_string = contents.split(',')
        decoded = base64.b64decode(content_string)
        df = pd.read_csv(io.StringIO(decoded.decode('utf-8')))
        # 在这里对上传的.csv文件进行处理，可以进行数据分析、可视化等操作
        return html.Div([
            html.H5(filename),
            html.H6('上传的文件内容：'),
            html.Pre(df.to_csv())
        ]), f'data:text/csv;base64,{base64.b64encode(decoded).decode()}'
    else:
        return html.Div(['拖放或点击选择.csv文件上传']), ''

if __name__ == '__main__':
    app.run_server(debug=True)