the file is an unknown format and cannot be opened

[#1]

I want to use another file Illustrator from some sources but I can noy open it in my computer .

I use Illustrator 2021 then I changed it to 2017 to solve it and tried many steps and can not solve it.

[#2]

What are those sources?

Is it really an Illustrator file? It starts with ._

[#3]

this file : ._artboards

[#4]

Yes, that is not an Illustrator file.

Do a search for ._ files

[#5]

I got some Illustrator files to practice and  I downloaded the files to my device, but unfortunately an error message appears the file is an unknown format and cannot be opened ز 

this file name : ._adobe_crests

[#6]

What are those sources?

I got it from courses in udemy . I donlowed lecturer udemy 

[#7 · Accepted solution]

The files should not start with ._

There should be similar files without the ._

That are the Illustrator files.

[#8]

Can I convert thus files?

[#9]

https://www.google.com/search?q=._+files