mime/multipart: restore 1.9 handling of missing/empty form-data file name

Revert the code changes of CL 96975 and CL 70931, but keep the tests,
appropriately modified for the code changes. This restores the 1.9
handling of form-data entries with missing or empty file names.

Changing the handling of this simply confused existing programs for no
useful benefit. Go back to the old behavior.

Updates #19183
Fixes #24041

Change-Id: I4ebc32433911e6360b9fd79d8f63a6d884822e0e
Reviewed-on: https://go-review.googlesource.com/121055
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/mime/multipart/formdata.go

@@ -58,7 +58,7 @@ func (r *Reader) readForm(maxMemory int64) (_ *Form, err error) {
 
 		var b bytes.Buffer
 
-		if !p.hasFileName() {
+		if filename == "" {
 			// value, store as string in memory
 			n, err := io.CopyN(&b, p, maxValueBytes+1)
 			if err != nil && err != io.EOF {

src/mime/multipart/formdata_test.go

@@ -47,12 +47,9 @@ func TestReadFormWithNamelessFile(t *testing.T) {
 	}
 	defer f.RemoveAll()
 
-	fd := testFile(t, f.File["hiddenfile"][0], "", filebContents)
-	if _, ok := fd.(sectionReadCloser); !ok {
-		t.Errorf("file has unexpected underlying type %T", fd)
+	if g, e := f.Value["hiddenfile"][0], filebContents; g != e {
+		t.Errorf("hiddenfile value = %q, want %q", g, e)
 	}
-	fd.Close()
-
 }
 
 func TestReadFormWithTextContentType(t *testing.T) {

src/mime/multipart/multipart.go

@@ -81,16 +81,6 @@ func (p *Part) FileName() string {
 	return p.dispositionParams["filename"]
 }
 
-// hasFileName determines if a (empty or otherwise)
-// filename parameter was included in the Content-Disposition header
-func (p *Part) hasFileName() bool {
-	if p.dispositionParams == nil {
-		p.parseContentDisposition()
-	}
-	_, ok := p.dispositionParams["filename"]
-	return ok
-}
-
 func (p *Part) parseContentDisposition() {
 	v := p.Header.Get("Content-Disposition")
 	var err error