try removing the underscore and try it again. I assume it has some sort of significance in the OSX file system.

At the user, GUI level Mac no issue with the underscore, and it is in some instances more preferable than the space. At any rate, I have tried to remove the underscore so the file name is just numbers, and it still produces the same error.

I have discovered an annoying way to get the downloads to work. If I try right-click the individual attachment, click Save As..., then a options box comes up (Save As name, Tags, Where and Format field). If I manually add .jpg to the Save As name then it works.

This is an unacceptable nuisance to me, give that: a) Thunderbird recognizes and lists the attachment as jpg at the bottom of the email, and; b) In the Save As... options box it also selects the Format as 'jpg'.

So why can't my Thunderbird just save the file as is, with the .jpg included in the filename?

I have found a solution in the settings.

Under Firefox>Settings>Attachments>Incoming there is a list of filetypes with a configurable choice of Actions. For all my filetypes, from the Actions options Always Ask, Save File, Use <program>, Use Other... -- mine are Always Asks. I changed my Preview Document (image/jpeg .jpg) to Save File.

I would interepret these options to be about how to deal with the email immediately upon receiving them from the server; namely the suggestion to Ask or Save doesn't make sense if I've already hit a Save As... button.

What does it have to do with adding .jpg manually to a filename that already has .jpg on it?

I am not closing the issue because I have seen this issue brought up by others, solved (or not) in other ways, so I believe it to be a bug worth fixing.