attachments do not exist as a file until Thunderbird writes then to the temp folder as part of the opening process. given you are finding the files, Thunderbird looks like it is doing it's part, but your operating system is not doing it's part for some reason. To work that out.

• Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.
• Restart the operating system in safe mode with Networking. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: Windows 8, Windows 7, Windows Vista, Windows XP, OSX

While in operating system safe mode try and start Thunderbird also in safe mode. Please report your results.

In addition to Matts advise. If you had previously detached the attachment and stored it in another file outside of thunderbird, then a place holder..link...to the stored location should be still working. The drop down menu would show 'Detach' and 'Delete' as greyed out - this would be normal, but 'Open' should still be available.

Select the email so you can see it in the Message Pane. click on 'More' button and select 'View Source' scroll down , do you see this header : X-Mozilla-External-Attachment-URL: It should give file location information where it was originally stored when detached. Do you see anything like this in the source? Have you since moved that detached attachment to another location ?