Our current installed versions are: Horde 5.2.8, IMP  6.2.11

If the editing message is saved as a draft then all attached files are saved.

If I go to Draft folder and I select the message then I can see all 
the attached files and these files can be downloaded or displayed by 
horde.

The error message appears with some file types just in editon mode, 
i.e. pdf, doc, png, jpg, but they can be displayed from a saved draft 
message.

So it appears this problem is caused by  compose message form

https://server/horde/imp/dynamic.php?page=compose&buid=14092&mailbox=RHJhZnRz&type=resume&token=whtkI_Z-D4Epp17lX9R-nkF&uniq=1461339533663

Some other extensions can be displayed in edition mode without any 
problem, i.e. txt, csv, sh.

The error message is always the same:

"A fatal error has occurred
Could not display attachment data.

1. IMP_Compose_View->composeAttachPreview() /var/www/horde/imp/view.php:45

Details
The full error message is logged in Horde's log file, and is shown 
below only to administrators. Non-administrative users will not see 
error details.

IMP_Exception Object
(
     [details] =>
     [logged] => 1
     [_logLevel:protected] => 0
     [message:protected] => Could not display attachment data.
     [string:Exception:private] =>
     [code:protected] => 0
     [file:protected] => /var/www/horde/imp/lib/Compose/View.php
     [line:protected] => 54
     [trace:Exception:private] => Array
         (
             [0] => Array
                 (
                     [file] => /var/www/horde/imp/view.php
                     [line] => 45
                     [function] => composeAttachPreview
                     [class] => IMP_Compose_View
                     [type] => ->
                     [args] => Array
                         (
                             [0] => 1
                             [1] => 1
                             [2] =>
                         )

                 )

         )

     [previous:Exception:private] =>
)
"

So what's the bug?  You haven't proven that the message data is 
actually being saved (i.e. your configuration is correct).  That's 
exactly the kind of message you should be receiving if the attachment 
data is not being saved correctly.

While editing a message if we upload an attachment and we try to click 
on the link of the attachment, we get this error message:

A fatal error has occurred
No es poden visualitzar les dades adjuntes.
( Translation: You can't visualize the attachment)


1. IMP_Compose_View->composeAttachPreview() /var/www/horde/imp/view.php:45

Details
The full error message is logged in Horde's log file, and is shown 
below only to administrators. Non-administrative users will not see 
error details.

IMP_Exception Object
(
     [details] =>
     [logged] => 1
     [_logLevel:protected] => 0
     [message:protected] => No es poden visualitzar les dades adjuntes.
     [string:Exception:private] =>
     [code:protected] => 0
     [file:protected] => /var/www/horde/imp/lib/Compose/View.php
     [line:protected] => 54
     [trace:Exception:private] => Array
         (
             [0] => Array
                 (
                     [file] => /var/www/horde/imp/view.php
                     [line] => 45
                     [function] => composeAttachPreview
                     [class] => IMP_Compose_View
                     [type] => ->
                     [args] => Array
                         (
                             [0] => 1
                             [1] => 1
                             [2] =>
                         )

                 )

         )

     [previous:Exception:private] =>
     [xdebug_message] => <tr><th align='left' bgcolor='#f57900' 
colspan="5"><span style='background-color: #cc0000; color: #fce94f; 
font-size: x-large;'>( ! )</span> IMP_Exception: No es poden 
visualitzar les dades adjuntes. in 
/var/www/horde/imp/lib/Compose/View.php on line <i>54</i></th></tr>
<tr><th align='left' bgcolor='#e9b96e' colspan='5'>Call Stack</th></tr>
<tr><th align='center' bgcolor='#eeeeec'>#</th><th align='left' 
bgcolor='#eeeeec'>Time</th><th align='left' 
bgcolor='#eeeeec'>Memory</th><th align='left' 
bgcolor='#eeeeec'>Function</th><th align='left' 
bgcolor='#eeeeec'>Location</th></tr>
<tr><td bgcolor='#eeeeec' align='center'>1</td><td bgcolor='#eeeeec' 
align='center'>0.0001</td><td bgcolor='#eeeeec' 
align='right'>239856</td><td bgcolor='#eeeeec'>{main}(  )</td><td 
title='/var/www/horde/imp/view.php' 
bgcolor='#eeeeec'>../view.php<b>:</b>0</td></tr>
<tr><td bgcolor='#eeeeec' align='center'>2</td><td bgcolor='#eeeeec' 
align='center'>0.0342</td><td bgcolor='#eeeeec' 
align='right'>2199216</td><td 
bgcolor='#eeeeec'>IMP_Compose_View->composeAttachPreview(  )</td><td 
title='/var/www/horde/imp/view.php' 
bgcolor='#eeeeec'>../view.php<b>:</b>45</td></tr>