Mac osx - strange extra files appearing when downloading files directly.

Thanks Daphne, 
Opened up a new browser (Firefox - initial browser was Chrome), and yes same exact issue. 
Downloading files from my one user (not shared) also produced the same results.
There was no need to clear browser cache on firefox since it was the first time I've ever visited DB on that browser.
Incognito window produced the same results as well (using different files too).
Thoughts?

Thank you for your time Daphne.
With respect though, I don't think this is all entirely accurate, in that I've always downloaded files directly to my system, and while I can't recall the exact date of the last time I did this, I know it would have been within the last month or so - which means it was working just a little while ago with my current OSX 10.11.6. So this tells me that it's something that Dropbox must have changed in how they zip the files. 
This will just be a frustrating glitch I guess from now on (until I can upgrade my system at some point.) 

One additional follow up...

I talked with my co-worker and he confirmed he's using 10.13.6, and he's having the same issue. 

Hey there JefW - sorry to jump in here; I just wanted to add my two cents on this.

I went through your problem's description and I saw that you mentioned that the files in question had an underscore within their names.

I'm going to take a guess here and say that they probably include a (.) dot too - right?

If I'm correct and that's indeed the case (you're seeing '._' within your files' names), I'll need to inform you that, unfortunately the issue you're seeing is a Mac issue that occurs when copying files with extended attributes metadata into a file system that doesn't support them. Depending on the information stored in the metadata, you may or may not need these files.

These files are called AppleDouble files and more information on how to handle them can be found by searching for: "apple double files"

To avoid this issue if you have your Dropbox folder on an external drive, we recommend ensuring that the drive is formatted in the default file system format for the Operating System that is installed (those would be HFS+ or APFS for Macs). 

Formatting drives may cause data to be deleted. Please consult resources from your Operating System's manufacturer (Apple in your case) for steps and recommendations on formatting drives to a different file system. The most common default file systems are NTFS (Windows), HFS (Mac OS X) or ext4 (Linux).

Let us know if this helped please!

Hello Walter, 
I do thank you for taking the time to reply. I'm afraid though that it still does not seem to apply to my (and my co-workers on a different computer) situation.
We both have our DB folders on our local computers, and not on External Drives.
The issue I was describing is when I do a 'direct download' to my desktop (also not an external drive, if you're familiar with macs). And what's most important here to remember... This is only a recent problem (within the last month)... ie: in the past whenever I did the direct download I never had an issue. Now for some reason, both mine and my co-workers imac are having the same problem. 
I assure you my imac HD is properly formatted.
But I thank you for your time. 
Just to add to the puzzle, this is only when I 'direct download' to my system and unzip the compressed files. I attempted a zip and decompress with other local files and had no issues, so it's something that Dropbox is doing to the zip files.

Thanks for the response Jef!

Now, kindly note that Dropbox does not have the capability of changing your files in any way. 

As I mentioned, if you're indeed seeing "._" within some of your files' names, these are usually the metadata of the original files (these are hidden normally) - more information on this can be found by a mere web search with the search string "Apple's Double Files".

If you're still having issues with this, please let me know and I'll reach out to your Dropbox associated email address to work on this together.

Thanks in advance and have a lovely day until we speak again. 

Thanks Walter,
Well, I know this is a technicality, but if I upload 2 files, and then choose to download them directly, Dropbox compresses those files into a zip file... which is changing my original files. The process of compression is what I meant by Dropbox changing the files.

Yes, my original issue is still unanswered. Just to re-state the issue...
When I do I direct download, and then uncompress the zipped files there are these extra files showing up. 
About a week or more (month maybe) it never used to do this.
Something recently changed - And I don't think it was my system, since it is also happening on my co-workers computer.

Thanks for your time.

Thanks for the additional information and sorry to hear this persists Jef.

At this point, as I'll need account specific information to help troubleshoot this issue, may I reach out to your Dropbox associated email address so we can have a further look into this matter?

In addition, have you checked if your device is showing 'hidden files'? If that's the case here, to avoid seeing hidden files:

defaults write com.apple.finder AppleShowAllFiles NO