Canvas not getting updated

Dash Python
1

Why is the canvas not getting updated when I supply another file as input?

import base64
import datetime
import io
import dash
from dash.dependencies import Input, Output, State
import dash_core_components as dcc
import dash_html_components as html
import dash_table
import dash_canvas
import dash_table
import pandas as pd
from dash_canvas.utils import parse_jsonstring_rectangle

external_stylesheets = ['https://codepen.io/chriddyp/pen/bWLwgP.css']

app = dash.Dash(__name__, external_stylesheets=external_stylesheets)

list_columns = ['width', 'height', 'left', 'top', 'label']
columns = [{'name': i, "id": i} for i in list_columns]
columns[-1]['presentation'] = 'dropdown'
list_preferred = ['Company Name','Company Address','Invoice Number','Tax','Total']
shortlists = [{'label': i, 'value': i} for i in list_preferred]

app.layout = html.Div([
    dcc.Upload(
        id='upload-data',
        children=html.Div([
            'Drag and Drop or ',
            html.A('Select Files')
        ]),
        style={
            'width': '100%',
            'height': '60px',
            'lineHeight': '60px',
            'borderWidth': '1px',
            'borderStyle': 'dashed',
            'borderRadius': '5px',
            'textAlign': 'center',
            'margin': '10px'
        },
        # Allow multiple files to be uploaded
        multiple=True
    ),
    dash_canvas.DashCanvas(
                            id='canvas',
                            width=500,
                            tool='rectangle',
                            lineWidth=2,
                            lineColor='rgba(255,0, 0, 0.5)',
                            hide_buttons=['pencil', 'line'],
                            goButtonTitle='Label'
                            ),
    html.Div([
        html.Div([
                html.H3('Label images with bounding boxes'),
                ]),
        html.Div([
                dash_table.DataTable(
                                    id='table',
                                    columns=columns,
                                    editable=True,
                                    dropdown = {'label': {'options': shortlists}},
                                    export_format='xlsx',
                                    export_headers='display'),
                ])
    ]),
    html.Div(id='output-data-upload'),
])
@app.callback(Output('canvas', 'filename'),[Input('upload-data', 'filename')],prevent_initial_call=True)
def update_output(list_of_names):
    print("list_of_names\n",list_of_names)
    filename = list_of_names
    if filename is not None and filename[0] !="":
        return app.get_asset_url(filename[0])


@app.callback(Output('table', 'data'), [Input('canvas', 'json_data')], [State('table','data')],prevent_initial_call=True)
def show_string(json_data,table_data):
    if json_data!=None:
        box_coordinates = parse_jsonstring_rectangle(json_data)
        df = pd.DataFrame(box_coordinates, columns=list_columns[:-1])
        stdt = df.to_dict('records')
        if table_data!=None:
            for i in range(len(table_data)):
                stdt[i]['label'] = table_data[i]['label']
        return stdt