displaying attachments inline does not affect the appearance of the attachment pane with attachments.

However it would appear that your mail is using the text attachment as a body as it no longer appears when you disable inline attachments.

Where is the mail coming from? many web apps and routers etc send appallingly formatted mail that is simply non standards compliant and you do battle to even read it. The most common problem is pig ignorant developers that are to lazy to look up and use the correct mime types for the attachment they use and think Mime type application/octet stream is the answer to all mime encoding. Thunderbird uses mime types to identify what type of file is attached.

However is you forward such a message to me (at an attachment from the message menu) at unicorn dot consulting at gmail dot com I will have a look and see if I can explain the behavior and offer a solution.

I have the same problem. In my case I attach a file with the extension pbn which is a special text file used in the world of bridge and the contents are displayed below the body of my message.

This is an example of an pbn file:

% Dealer4 ver 4.54 [Event "QBDB"] [Site "Qualicum Beach"] [Date "July 28, 2014"] [Board "1"] [Dealer "N"] [Vulnerable "None"] [Deal "N:A982.4.J862.J652 K6.K87653.AT3.93 JT73.T9.Q97.K874 Q54.AQJ2.K54.AQT"] [DoubleDummyTricks "1111111111b7b97b7b97"] [OptimumResultTable "Declarer;Denomination\2R;Result\2R"] N S 1

what same problem? no message body, no attachment pane or just what?

I do not recall getting an email from the original poster, so I am no more informed than when I posted this a month ago.