What I have found is
If the attachment is a zip file, don't download, open the zip file and then drag the contents to your machine
If the attachment is not zip file, Excel seems to copy across OK, but for access, save as and rename the extension as .accdb/.mdb as appropriate - or once saved, rename the saved file. In both cases the file is called attachment, so you can rename this at the same time if required.
It is a bit of a pain, but other than images, I've taken to uploading all files as zip files