the 'body' value in the dictionary should be the base64 encoded contents of the file, not the file name. you need to read and encode the file contents yourself. e.g.
body = ""
with open("/Users/My_Files/untitled.txt", "rb") as f:
body = f.read().encode("base64")
update_dict = {
'type' : 'Attachement'
'ParentId' : accountId,
'Name' : 'untitled.txt',
'Body' : body }
...
Docs about Attachment