.htaccess rewrite full domain name
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Are there anyway to make a user get to download thefile by let them type:`download.mywebsite.com/thefile` 
in the browser address while the "download.mywebsite.com/thefile" does not exist but the "www.mywebsite.com/thefile" does.

I've got nearly zero knowledge with .htaccess and mod_rewrite engine. I hope my question is clear.