This implements 'attachment' mode triggered then
?filename parameter is accompanied with &download=true
When Content-Disposition: attachment is detected by a modern browser
it will skip rendering and immediately open the "save as" dialog,
making this useful feature for using IPFS gateway as target of
"Download" links on various websites.
Parameter name was suggested in:
https://github.com/ipfs/go-ipfs/pull/4177#issuecomment-414870327
