I came across this same "bug" when testing an ebook on Mail that I'm revising for Mac OS X 10.5. Sometimes the attachment saved and sometimes it didn't. Here's the solution for saving attachments (and a preview of my ebook):
Saving Attachments
File Menu
One way to save attachments is to highlight or open an email message that contains an attachment and select Save Attachments... from the File menu. A Save pane will open and you can select a location for the saved attachment. If the message contains more than one attachment, all attachments will be saved in the selected location. Note that you can change the name of the saved file for single attachments but multiple attachments will be saved with their original file names.
Save Button
Another way to save an attachment is to highlight the email message with the attachment in the Message List and make sure the Preview Pane is visible. Click and hold the Save button that appears in the header of the Preview Pane and select the attachment you want to save from the drop-down menu. Note that with this option, you can select to save all attachments or just one specific attachment.
Contextual Menu
Open an email message with an attachment and control-click (right-click) on an attachment and select the option you want from the contextual menu:
• Save Attachment... - A Save pane will open and you can select a location for the saved attachment. This option also lets you rename the attachment.
• Save to Downloads Folder - Mail will save the attachment to the Mail downloads folder location.
Drag & Drop
Another option for saving an attachment is to open an email message that contains an attachment and drag the attachment to the desktop or another location on your hard drive. If the attachment is in an IMAP mailbox, you may need to be online to drag the attachment, depending on whether or not the attachment was downloaded to Mail before you attempt to drag it.
The solutions are not perfect because the attachments will be saved without the message. Hopefully, this will be corrected in Mac OS X 10.5.2.
Claire