Email not sending out excel attachment

I am trying to send out emails with attachments to various recipients stored in a csv file. My aim is to send the recipients the corresponding attachment which is stored in a specific location.

The csv looks like this

Name,Emails,Firm
Tom,tom@fakedomian.com,Firm1
Dick,dick@fakedomain.com,Firm2
Harry,harry@fakedomain.com,Firm3

Within the folder the attachments would look like this

path='C:\\Documents\\Firms'

Within the location the files would look as follows

Firm1.xlsx
Firm2.xlsx
Firm3.xlsx

The following code uses the name and email in the csv to send out a personalised email while going through the folder to attach the corresponding excel file.

import email, csv, smtplib, ssl, fnmatch, os, codecs

from email import encoders
from email.mime.base import MIMEBase
from email.mime.multipart import MIMEMultipart
from email.mime.text import MIMEText
import fnmatch

path = 'C:\\Documents\\Firms'
email_html = open('email.html')
body = email_html.read()


from_address = "youremail@domain.com"
password = input("Type your password and press enter: ")

#context = ssl.create_default_context()
with smtplib.SMTP("smtp.office365.com", 587) as server:
    server.ehlo()
    server.starttls()
    server.login(from_address, password)
    with open("Firms.csv") as file:
        reader = csv.reader(file)
        next(reader)
        for Name, Email, Firm in reader:
            # Create a multipart message and set headers
            message = MIMEMultipart()
            message["From"] = from_address
            message["To"] = Email
            message["Subject"] = 'Test ' +  Name

            #Add body to email and excel attachment

            message.attach(MIMEText(body, "html"))

            for i in os.listdir(path):
                if os.path.isfile(os.path.join(path,i)) and Firm in i:
                    with open(os.path.join(path, i), 'rt', encoding='ISO-8859-1') as attachment:
                        part = MIMEBase("application", "octet-stream")
                        part.set_payload(attachment.read())

                        encoders.encode_base64(part)

                        part.add_header(
                        "Content-Disposition",f"attachment; filename= {Firm}",)

                    message.attach(part)
                    text = message.as_string()

                    # Use server to send email
                    server.sendmail(from_address, Email, text.format(name=Name))

Although this sends out emails the attachments are not in an excel format. When I used a encoding='utf8' I would get following error

UnicodeDecodeError: 'charmap' codec can't decode byte 0x81 in position 18: character maps to <undefined>

Any help here would be much appreciated.