mime/multipart: add helper to build content-disposition header contents

api/next/46771.txt

@@ -0,0 +1 @@
+pkg mime/multipart, func FileContentDisposition(string, string) string #46771

src/mime/multipart/writer.go

@@ -139,9 +139,7 @@ func escapeQuotes(s string) string {
 // a new form-data header with the provided field name and file name.
 func (w *Writer) CreateFormFile(fieldname, filename string) (io.Writer, error) {
 	h := make(textproto.MIMEHeader)
-	h.Set("Content-Disposition",
-		fmt.Sprintf(`form-data; name="%s"; filename="%s"`,
-			escapeQuotes(fieldname), escapeQuotes(filename)))
+	h.Set("Content-Disposition", FileContentDisposition(fieldname, filename))
 	h.Set("Content-Type", "application/octet-stream")
 	return w.CreatePart(h)
 }
@@ -155,6 +153,13 @@ func (w *Writer) CreateFormField(fieldname string) (io.Writer, error) {
 	return w.CreatePart(h)
 }
 
+// FileContentDisposition returns the value of a Content-Disposition header
+// with the provided field name and file name.
+func FileContentDisposition(fieldname, filename string) string {
+	return fmt.Sprintf(`form-data; name="%s"; filename="%s"`,
+		escapeQuotes(fieldname), escapeQuotes(filename))
+}
+
 // WriteField calls CreateFormField and then writes the given value.
 func (w *Writer) WriteField(fieldname, value string) error {
 	p, err := w.CreateFormField(fieldname)

src/mime/multipart/writer_test.go

@@ -172,3 +172,22 @@ func TestSortedHeader(t *testing.T) {
 		t.Fatalf("\n got: %q\nwant: %q\n", buf.String(), want)
 	}
 }
+
+func TestFileContentDisposition(t *testing.T) {
+	tests := []struct {
+		fieldname string
+		filename  string
+		want      string
+	}{
+		{"somefield", "somefile.txt", `form-data; name="somefield"; filename="somefile.txt"`},
+		{`field"withquotes"`, "somefile.txt", `form-data; name="field\"withquotes\""; filename="somefile.txt"`},
+		{`somefield`, `somefile"withquotes".txt`, `form-data; name="somefield"; filename="somefile\"withquotes\".txt"`},
+		{`somefield\withbackslash`, "somefile.txt", `form-data; name="somefield\\withbackslash"; filename="somefile.txt"`},
+		{"somefield", `somefile\withbackslash.txt`, `form-data; name="somefield"; filename="somefile\\withbackslash.txt"`},
+	}
+	for i, tt := range tests {
+		if found := FileContentDisposition(tt.fieldname, tt.filename); found != tt.want {
+			t.Errorf(`%d. found: "%s"; want: "%s"`, i, found, tt.want)
+		}
+	}
+}