What do you mean by "viewing the attachment" ? Read the document or see an icon?

i need to view the attachment before send the email, in thunderbird i attached a file and before send it i click on attachement directly in thunderbird (not in filesystem) and i see it.
I need to read the document.

Actually, the duplicate (0001321) was closed but did not assess the issue mentioned in this bug. The right-click menu in the AttachmentArea has an option to "Open" an attachment after inserting it into the attachment list while composing a new mail, but this option is greyed out.
The attachment list items could perhaps be presented as URLs ("file:///...") rather than as plain text entries to enable the user to locally open and review attachments he has added to the mail before sending. Alternatively, double-clicking on an attachment list item or selecting "Open" from the context menu of an item should open the corresponding file.

There is no way to show an attachment for a message in composition as the file is still on the file system and cannot be "redownloaded". Also, the SOGo JS code does not have access to the local filesystem either. Even when a filename is specified, recent browsers will hide the current file location, so this is physically not possible to implement this.