feat: use ContentType to control HTML extension

internal/protocol/gopher/gopher.go

@@ -209,9 +209,22 @@ func (p *Protocol) downloadText(ctx context.Context, reader *bufio.Reader, req *
 		IPVersion:       4,
 		Speed:           speed,
 		OutputPath:      req.Output,
+		ContentType:     gopherContentType(firstType),
 	}, nil
 }
 
+// gopherContentType returns the MIME content type for a gopher item type.
+func gopherContentType(t byte) string {
+	switch t {
+	case typeHTML:
+		return "text/html"
+	case typeTextFile, typeInformation:
+		return "text/plain"
+	default:
+		return "application/octet-stream"
+	}
+}
+
 // downloadDirectory formats a directory listing as readable text.
 func (p *Protocol) downloadDirectory(reader *bufio.Reader, req *core.DownloadRequest, startTime time.Time) (*core.DownloadResult, error) {
 	var writer io.Writer
@@ -261,6 +274,7 @@ func (p *Protocol) downloadDirectory(reader *bufio.Reader, req *core.DownloadReq
 		IPVersion:       4,
 		Speed:           speed,
 		OutputPath:      req.Output,
+		ContentType:     "text/plain",
 	}, nil
 }
 
@@ -313,6 +327,7 @@ func (p *Protocol) downloadBinary(ctx context.Context, reader *bufio.Reader, req
 		IPVersion:       4,
 		Speed:           speed,
 		OutputPath:      req.Output,
+		ContentType:     "application/octet-stream",
 	}, nil
 }