Recover from possible infection by virus attachment via email
GENERALLY speaking just downloading a file or saving it to your hard drive will not infect your Mac. Generally.

What you have done after inadvertently saving the file is trashing it and then emptying the trash, which is exactly what you should have done. While it is possible that whatever you downloaded infected your Mac, I would say the odds are pretty low.

There are a number of free or inexpensive malware scanners available for macOS. Malwarebytes and AVG spring to mind, but there are others and perhaps others that do a better job. Wander over to macworld.com (or just search on AV software for Mac) and see what they say about Mac AV software. Then download and use the one that looks good to you.