To help avoid this problem in the furture, and if you have room on your computer, save the original file in a folder just for the originals. Then rename them.
If you also downloaded the picture/photo of the stitchout, you can drag-and-drop it in Google Search box for Images. USUALLY, that will give you where the picture can be found, which will also have the digitized file/designer/website. Hope you fine a few this way. SOMTIMES just a short description of the design in Google will find it for you(and select Images). Even if you don't post links, you might want to know for yourself!
I always try to download a picture too. I have a few designs that I missed getting the digitizer/website info for. I will try your search idea to see if I can finally identify where these designs came from. Thank you for this idea!
When I download designs, I try to always add the digitizer or website name to the design name if it is not already included. Then, if and when I finally get around to using the design, I can give them credit or ask questions if I have a problem. Not much help for designs you have already downloaded, but you might give a thought to doing this in the future.
In some cases you should not put a link for example, any website that offers illegal designs... do not put a link to that site and do not even mention the site in your post. We remove all posts that have links to illegal sites.
You are not required to add a link. Some may ask, if you remember...great. If not, no worries. Simply tell them you don't remember.