feat(gateway): ?filename with download=true

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:
#4177 (comment)

core/corehttp/gateway_handler.go

@@ -261,7 +261,11 @@ func (i *gatewayHandler) getOrHeadHandler(w http.ResponseWriter, r *http.Request
 		urlFilename := r.URL.Query().Get("filename")
 		var name string
 		if urlFilename != "" {
-			w.Header().Set("Content-Disposition", fmt.Sprintf("inline; filename*=UTF-8''%s", url.PathEscape(urlFilename)))
+			disposition := "inline"
+			if r.URL.Query().Get("download") == "true" {
+				disposition = "attachment"
+			}
+			w.Header().Set("Content-Disposition", fmt.Sprintf("%s; filename*=UTF-8''%s", disposition, url.PathEscape(urlFilename)))
 			name = urlFilename
 		} else {
 			name = getFilename(urlPath)

docs/gateway.md

@@ -49,6 +49,16 @@ your query string to explicitly specify the filename. For example:
 
 > https://ipfs.io/ipfs/QmfM2r8seH2GiRaC4esTjeraXEachRt8ZsSeGaWTPLyMoG?filename=hello_world.txt
 
+When you try to save above page, you browser will use passed `filename` instead of a CID.
+
+## Downloads
+
+It is possible to skip browser rendering of supported filetypes (plain text,
+images, audio, video, PDF) and trigger immediate "save as" dialog by appending
+`&download=true`:
+
+> https://ipfs.io/ipfs/QmfM2r8seH2GiRaC4esTjeraXEachRt8ZsSeGaWTPLyMoG?filename=hello_world.txt&download=true
+
 ## MIME-Types
 
 TODO

test/sharness/t0110-gateway.sh

@@ -31,11 +31,16 @@ test_expect_success "GET IPFS path succeeds" '
   curl -sfo actual "http://127.0.0.1:$port/ipfs/$HASH"
 '
 
-test_expect_success "GET IPFS path with explicit filename succeeds with proper header" "
+test_expect_success "GET IPFS path with explicit ?filename succeeds with proper header" "
   curl -fo actual -D actual_headers 'http://127.0.0.1:$port/ipfs/$HASH?filename=testтест' &&
   grep -F \"Content-Disposition: inline; filename*=UTF-8''test%D1%82%D0%B5%D1%81%D1%82\" actual_headers
 "
 
+test_expect_success "GET IPFS path with explicit ?filename and download=true succeeds with proper header" "
+  curl -fo actual -D actual_headers 'http://127.0.0.1:$port/ipfs/$HASH?filename=testтест&download=true' &&
+  grep -F \"Content-Disposition: attachment; filename*=UTF-8''test%D1%82%D0%B5%D1%81%D1%82\" actual_headers
+"
+
 # https://github.com/ipfs/go-ipfs/issues/4025#issuecomment-342250616
 test_expect_success "GET for Service Worker registration outside of an IPFS content root errors" "
   curl -H 'Service-Worker: script'  -svX GET 'http://127.0.0.1:$port/ipfs/$HASH?filename=sw.js' > curl_sw_out 2>&1 &&