diff --git a/utils/str/content_disposition.go b/utils/str/content_disposition.go
index 22a28fdbd..34ae8a5c6 100644
--- a/utils/str/content_disposition.go
+++ b/utils/str/content_disposition.go
@@ -12,7 +12,11 @@ import (
 	"github.com/deluan/sanitize"
 )
 
-const maxFilenameBytes = 255
+const (
+	maxFilenameBytes  = 255
+	maxExtensionBytes = 16
+	fallbackFilename  = "download"
+)
 
 // ContentDispositionAttachment builds an RFC 6266 attachment header value for a user-controlled filename.
 // Non-ASCII names also get a filename* parameter, which clients prefer over the ASCII fallback.
@@ -36,8 +40,11 @@ func splitFilename(filename string) (stem, ext string) {
 		return r
 	}, SanitizeFilename(strings.ToValidUTF8(filename, "_")))
 	ext = path.Ext(name)
+	if len(ext) > maxExtensionBytes {
+		ext = ""
+	}
 	stem = strings.TrimSuffix(name, ext)
-	if limit := max(maxFilenameBytes-len(ext), 0); len(stem) > limit {
+	if limit := maxFilenameBytes - len(ext); len(stem) > limit {
 		for limit > 0 && !utf8.RuneStart(stem[limit]) {
 			limit--
 		}
@@ -47,8 +54,8 @@ func splitFilename(filename string) (stem, ext string) {
 }
 
 func joinFilename(stem, ext string) string {
-	stem = cmp.Or(strings.TrimSpace(stem), "download")
-	return strings.TrimRight(stem+ext, " .")
+	stem = cmp.Or(strings.TrimSpace(stem), fallbackFilename)
+	return cmp.Or(strings.TrimRight(stem+ext, " ."), fallbackFilename)
 }
 
 func toASCII(s string) string {
diff --git a/utils/str/content_disposition_test.go b/utils/str/content_disposition_test.go
index 2a768a1ca..944392a4e 100644
--- a/utils/str/content_disposition_test.go
+++ b/utils/str/content_disposition_test.go
@@ -61,6 +61,16 @@ var _ = Describe("ContentDispositionAttachment", func() {
 			Expect(asciiName("Loose End. ")).To(Equal("Loose End"))
 		})
 
+		It("falls back to a placeholder when only dots are left", func() {
+			Expect(str.ContentDispositionAttachment("...")).To(Equal(`attachment; filename="download"`))
+		})
+
+		It("caps a long name whose last dot is not an extension", func() {
+			name := asciiName("a." + strings.Repeat("x", 300))
+			Expect(len(name)).To(BeNumerically("<=", 255))
+			Expect(name).To(HavePrefix("a.xxx"))
+		})
+
 		It("replaces path separators and reserved characters", func() {
 			Expect(asciiName("AC/DC: Live, 1979?.zip")).To(Equal("AC_DC_ Live_ 1979_.zip"))
 		})