Before you start you need to identify the type of file you have attached. It is not "scanned document" Brother, the maker of my printer offer a number of choices. See https://support.brother.com/g/b/faqend.aspx?c=us_ot&lang=en&prod=mfc8952dw_us&faqid=faq00000046_004

Once you have identified the type of file you can move on to what application opens that type of file. Noting here that the file extension has application to file types only in Windows, not in other operating systems and not in internet documents of which email is one, web pages are another. Hence my comments about some cheap printers below.

Note that many cheap printers have no idea how to attached a document to an email correctly and attach them as text or some for of format flowed binary. Thunderbird will not be able to open them successfully because they are not correctly described. But lets first work out what file format they think they are using.

Hi Matt, Al Scanned Documents are in PDF format & in high Quality very accassionly,I wiil need to add a JPG file Where can I alter the preferences in Thunderbird Thanks Darryl

Sound to me like your AI might be hallucinating. Thunderbird handles PDF files internally. Can you open the attachment from the sent mail folder after it is sent? Or do you get the same error?

Hi Matt,Thank you for your assistance Yes after sending the mails.I can open the attachments ok The problem creates before I send the email I just need to open the attachments,to check & make sure it is the correct attachments thanks Darryl

So no hallucinations then I guess. But if I attach a JPG to an email I am writing, a double click opens it. Attach a PDF and it also opens. In a new tab in Thunderbird. That would indicate there either the formatting of the email you have is wrong (that hallucination) or you have never opened a JPG in Thunderbird. A PDF comes predefined, hence my suggestion that your search on that below

What is showing in settings for helper applications. (search pdf)

Hi Matt,I went into settings,but no application helper

Hi Matt,I went into settings,but no application helper

I told you to search for PDF. It is even shown in the search box of the image I posted. Please do as asked if you want to locate the appropriate settings.

Hi Matt Sorry attached is screen shot