Merge PR #1495 latest head: infer extension for opaque URLs

internal/protocol/http/fetcher.go

@@ -459,6 +459,7 @@ func (f *Fetcher) Resolve(req *base.Request, opts *base.Options) error {
 	if file.Name == "" || file.Name == "/" || file.Name == "." {
 		file.Name = httpReq.URL.Hostname()
 	}
+	file.Name = appendFilenameExtension(file.Name, resp.Header.Get("Content-Type"))
 
 	res.Files = append(res.Files, file)
 	f.meta.Res = res

internal/protocol/http/fetcher_test.go

@@ -221,6 +221,30 @@ func TestFetcher_Resolve(t *testing.T) {
 	})
 }
 
+func TestFetcher_ResolveFilenameFromContentType(t *testing.T) {
+	server := httptest.NewServer(gohttp.HandlerFunc(func(w gohttp.ResponseWriter, r *gohttp.Request) {
+		w.Header().Set("Content-Type", "image/webp")
+		w.WriteHeader(gohttp.StatusOK)
+	}))
+	defer server.Close()
+
+	fetcher := buildFetcher()
+	defer fetcher.Pause()
+
+	const opaqueName = "OIP-C.u5nEXTXND3PJF0gNTgw5YgHaFP"
+	err := fetcher.Resolve(&base.Request{URL: server.URL + "/" + opaqueName}, &base.Options{
+		Name: test.DownloadName,
+		Path: test.Dir,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if got := fetcher.meta.Res.Files[0].Name; got != opaqueName+".webp" {
+		t.Fatalf("resolved filename = %q, want %q", got, opaqueName+".webp")
+	}
+}
+
 func TestFetcher_ResolveWithHostHeader(t *testing.T) {
 	listener := test.StartTestHostHeaderServer()
 	defer listener.Close()

internal/protocol/http/helper.go

@@ -12,6 +12,7 @@ import (
 	"net/http"
 	"net/http/cookiejar"
 	"net/url"
+	"path"
 	"strings"
 	"time"
 	"unicode/utf8"
@@ -314,6 +315,28 @@ func parseFilename(contentDisposition string) string {
 	return parseFilenameFallback(contentDisposition)
 }
 
+// appendFilenameExtension adds a MIME-derived extension when the resolved
+// filename has no usable extension. Some image hosts use opaque path segments
+// that look like extensions, even though the response Content-Type is the
+// only reliable source for the file type.
+func appendFilenameExtension(filename, contentType string) string {
+	mediaType, _, err := mime.ParseMediaType(contentType)
+	if err != nil {
+		return filename
+	}
+
+	extensions, err := mime.ExtensionsByType(mediaType)
+	if err != nil || len(extensions) == 0 {
+		return filename
+	}
+
+	ext := path.Ext(filename)
+	if len(ext) > 1 && len(ext) <= 20 {
+		return filename
+	}
+	return filename + extensions[0]
+}
+
 // parseFilenameExtended handles RFC 5987 extended notation (filename*=)
 // Format: filename*=charset'language'value (e.g., UTF-8”%E6%B5%8B%E8%AF%95.zip)
 func parseFilenameExtended(cd string) string {