Admin meta box upload file (pdf or excel)

I can’t find info on how to do this anywhere.

I want a button in admin that the admin user can click to upload a file. The meta box will save the file. I can’t even find a plugin that will do this.

Any ideas?

So far I’ve tried this, but it wipes the file if you make any other changes and click on Update.

http://wordpress.stackexchange.com/questions/241097/meta-box-file-upload-wipes-uploaded-file-on-save