Need to replace a string with a file (HTML file)

[tester_V]

Greetings!
I need to send tons of emails, to do that I have to use an HTML file.
The string “ MessageBody” in the HTML file must be replaced with the Message file which has about 250 words and some coma delimited data that .csv file. I thought I could/should use 'fileinput' but I might be wrong.
Here is a snipped for the task and it does not work, error:
replace() argument 2 must be str, not _io.TextIOWrapper

import fileinput
relace_with = open('C:/01/File_replace/Email_Body.txt','r') 
relace_with.close()  
with fileinput.FileInput('C:/01/File_replace/Template_EM.html') as file:
    for line in file:
        line = line.replace("Message", relace_with)
        print(line)
        with open('C:/01/File_replace/Template_EM_New.html','w') as nf :
            nf.write(f"line\n")        
exit            

Any help is appreciated.
Thank you

[Larz60+]

the following assumes the actual text you want to replace is "Message"
wrote on the fly, I have not tested this code

import fileinput


with open('C:/01/File_replace/Email_Body.txt','r') as infile:
    replacement_text = infile.read()

with open('C:/01/File_replace/Template_EM.html') as currentfile, 
    open('C:/01/File_replace/Template_EM_New.html','w') as nf:

    for line in currentfile:
        if "Message" in line:
            line.replace("Message", replacement_text)
        nf.write(line)