add_attech method with file suffix '.txt' error

[targetoyes]

add_attech method with file’s suffix like '.txt' will be called something like this
File "/usr/lib/python2.6/site-packages/envelopes/envelope.py", line 329, in send
send_result = conn.send(self)
File "/usr/lib/python2.6/site-packages/envelopes/conn.py", line 87, in send
msg = envelope.to_mime_message()
File "/usr/lib/python2.6/site-packages/envelopes/envelope.py", line 292, in to_mime_message
msg.attach(MIMEText(part[1], type_min, self._charset))
File "/usr/lib64/python2.6/email/mime/text.py", line 30, in init
self.set_payload(_text, _charset)
File "/usr/lib64/python2.6/email/message.py", line 224, in set_payload
self.set_charset(charset)
File "/usr/lib64/python2.6/email/message.py", line 268, in set_charset
self._payload = charset.body_encode(self._payload)
File "/usr/lib64/python2.6/email/charset.py", line 387, in body_encode
return email.base64mime.body_encode(s)
File "/usr/lib64/python2.6/email/base64mime.py", line 147, in encode
enc = b2a_base64(s[i:i + max_unencoded])
File "/usr/lib64/python2.6/email/message.py", line 292, in getitem
return self.get(name)
File "/usr/lib64/python2.6/email/message.py", line 358, in get
name = name.lower()
AttributeError: 'slice' object has no attribute 'lower'

other suffix seems ok.

[thakkarparth007]

Please see #18. I've posted the fix there. I'll probably send a PR soon.

[thakkarparth007]

My bad. There's already a PR doing that. (#28)

[targetoyes]

ok，thanks