What is the files in the download folder?

[Havouza]

What is the files in the download folder?

over 400 files over 500 mb. 

All named like 0005d5d206c2458c0ef5b5e30cafa907e46d54bf

oldest from beginning 2017

Can be deleted?

[datakick]

Directory that contains Attachments and/or Downloadable products.

[datakick]

You can look into your database here:

select * from tb_attachment;

and here

select * from tb_product_download;

to see what files should exists. You can (probably) delete any files that are not listed there -- unless some module is using the same directory to store its own assets.