I'm not sure I understand what you're asking, but here's a thought:

When you have attached a file to a message, the way to re-check your attachment is to open the original file from its original location and check it. Trying to open it from the unsent message doesn't work. After you've sent the message, the attachment is viewable with the message, in the Sent folder. Before you send, carefully check the files you plan to send as attachments, and then attach them to your outgoing message, confident that they are what you intended to send.

The Message Box state they can't be opened - because the Associated helper Application Does Not Exist. I could all way open them before sending to check them, it changed a few month a go and I'm at a loss at what to do . I'm a voluntary Researcher of Commonwealth Veterans I'm 81 and I'm pulling what Hear I have out trying trying to figure the problem out.

Its the best e mail Program I'v had, its uncomplicated easy to use and extremely reliable I tried to re -installed Thunderbird to day - same problem Thank You Chris

Chris,

Looks as though you are trying to open and check an attachment in your unsent mail. Looks as though you're attaching a file to an unsent (yet) message, and then trying to open that attachment to check it. I don't think that's the way to do the checking. Check the file before you attach it, then attach it, and later, after you send the message, you can check the Sent Mail folder to find your message and make sure the attachment you sent is correct.

Sorry I can't help you with the problem the way you presented it. Maybe I just don't understand.