Changes have been made in Git for this ticket:

Bug #9245: Link attachments if guessed MIME type has a viewer

http://git.horde.org/diff.php/imp/lib/Contents.php?rt=horde-git&r1=4d0876c879537ec742fa80557716bf88874e3a7b&r2=eaf0ec8bbaaeb1405863771dcc7d9e44f05072c8

Chuck's commit c29f138bb9e84f6a73b85e3f1dd5cd451b2ddf1b fixed the mime 
type detection, but those files are still not linked for viewing 
through Horde_Mime_Viewer.

I meant for message attachments. I have the same message in both IMP 4 
and 5. It has 3 attachments (1 PDF, 2 iCalendar) as 
application/octet-stream but they are only displayed as PDF and 
iCalendar attachments in IMP 4.

For compose attachments?  Yes we do, in the compose_attach hook.

For message attachments?  Yes we do, in IMP_Contents::getSummary().   
Although we only do filename guessing - it is too expensive for us to 
have to download the entire contents of a potentially very large file 
from the IMAP server to do a fileinfo-type guess.  Although I guess we 
could now leverage the power of the IMAP server and only download a 
certain portion of the file (i.e. first 1000 bytes) to do fileinfo 
guessing, although I don't know enough about the heuristics to know if 
it only does scanning at the beginning of data.

.