Давайте поддерживать порядок, тогда шансы решить проблему существенно возрастут.

*** This bug has been marked as a duplicate of bug 20910 ***

(В ответ на комментарий №0)

> 1) when trying to open attachments, it never has a predefined program to open
> that type of files (I have tested it on common file types such as .doc, .pdf,
> .xls, .rtf, .ppt).

And for an .html attachment it wanted to use links, which is not the best choice for a graphical DE (Xfce).

> 2) I have to choose the program by choosing the executable file in /usr/bin/ --

and I had to choose /usr/bin/seamonkey by hand, which took quite a long time (browsing the filesystem through the file selection dialog).

> this is quite inconvenient and also almost impossible for a novice users: a
> menu-like application chooser would be more convenient.

And it's impossible to select a program to open the attachment, if it has no correct specific type declared; like this one:

Content-Type: application/octet-stream;
 name="Priobretenie_marsrutizatora.docx"
Content-Transfer-Encoding: base64
Content-Disposition: attachment;
 filename="Priobretenie_marsrutizatora.docx"

Only saving, and opening through the file manager (which has a working correct association for ".docs") succeeds.

So, it's a wanted feature: thunderbird suggests a program for opening an attachment also based on the attachment's filename (extension).