mime: in globs2 file only keep first time extension is seen

Fixes #45534

Change-Id: I9855607e845951f26ab85cb179ec6dea40d92156
Reviewed-on: https://go-review.googlesource.com/c/go/+/309574
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>

src/mime/testdata/test.types.globs2

@@ -6,3 +6,4 @@
 # mime package test for globs2
 50:document/test:*.t3
 50:example/test:*.t4
+30:example/do-not-use:*.t4

src/mime/type_unix.go

@@ -49,7 +49,15 @@ func loadMimeGlobsFile(filename string) error {
 			continue
 		}
 
-		setExtensionType(fields[2][1:], fields[1])
+		extension := fields[2][1:]
+		if _, ok := mimeTypes.Load(extension); ok {
+			// We've already seen this extension.
+			// The file is in weight order, so we keep
+			// the first entry that we see.
+			continue
+		}
+
+		setExtensionType(extension, fields[1])
 	}
 	if err := scanner.Err(); err != nil {
 		panic(err)