When handling email messages with text/plain file attached, it's contents is attached to issue description text (file from email is also properly attached to newly created issue, as it should be).

I believe that it may be caused by patch #13646, which combines all text/plain message parts to issue description and not distinguish between text part and file attachment?.

Example of problematic mail (sent from Thunderbird client):

--===============0191659920==
Content-Type: text/plain; charset="utf-8"
MIME-Version: 1.0
Content-Transfer-Encoding: base64

SGzDocWhZW7DrSBvIGNoeWLEmyB2IHByb2dyYW11IEhPUkVTCj09PT09PT09PT09PT09PT09PT09
PT09PT09PT09PT09CgpEYXR1bTogMTEuMTIuMjAxMy... proper message body text continues

--===============0191659920==
Content-Type: text/plain; charset="utf-8"
MIME-Version: 1.0
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="log_message.txt"

RGF0ZTogICAgMTEuMTIuMjAxMyAxODoxODowMQpNZXNzYWdlOiBBdHRyaWJ1dGVFcnJvcjogJ2Z1
bmN0aW9uJyBvYmplY3QgaGFzIG5vIGF0dHJpYnV0ZSAnaXNEZWFkJwpUeXBlOiAgICA0C... attachment data continues

Problems started when upgraded to Redmine 2.4.0