Infer content type in alloc fs stat endpoint

client/allocdir/alloc_dir.go

@@ -11,6 +11,9 @@ import (
 	"sync"
 	"time"
 
+	"net/http"
+	"strings"
+
 	hclog "github.com/hashicorp/go-hclog"
 	multierror "github.com/hashicorp/go-multierror"
 	cstructs "github.com/hashicorp/nomad/client/structs"
@@ -392,15 +395,41 @@ func (d *AllocDir) Stat(path string) (*cstructs.AllocFileInfo, error) {
 		return nil, err
 	}
 
+	contentType := detectContentType(info, p)
+
 	return &cstructs.AllocFileInfo{
-		Size:     info.Size(),
-		Name:     info.Name(),
-		IsDir:    info.IsDir(),
-		FileMode: info.Mode().String(),
-		ModTime:  info.ModTime(),
+		Size:        info.Size(),
+		Name:        info.Name(),
+		IsDir:       info.IsDir(),
+		FileMode:    info.Mode().String(),
+		ModTime:     info.ModTime(),
+		ContentType: contentType,
 	}, nil
 }
 
+// detectContentType tries to infer the file type by reading the first
+// 512 bytes of the file. Json file extensions are special cased.
+func detectContentType(fileInfo os.FileInfo, path string) string {
+	contentType := "unknown"
+	if !fileInfo.IsDir() {
+		f, err := os.Open(path)
+		// Best effort content type detection
+		// We ignore errors because this is optional information
+		if err == nil {
+			fileBytes := make([]byte, 512)
+			_, err := f.Read(fileBytes)
+			if err == nil {
+				contentType = http.DetectContentType(fileBytes)
+			}
+		}
+	}
+	// Special case json files
+	if strings.HasSuffix(path, ".json") {
+		contentType = "application/json"
+	}
+	return contentType
+}
+
 // ReadAt returns a reader for a file at the path relative to the alloc dir
 func (d *AllocDir) ReadAt(path string, offset int64) (io.ReadCloser, error) {
 	if escapes, err := structs.PathEscapesAllocDir("", path); err != nil {

client/allocdir/alloc_dir_test.go

@@ -472,3 +472,18 @@ func TestPathFuncs(t *testing.T) {
 		t.Errorf("%q is not empty. empty=%v error=%v", dir, empty, err)
 	}
 }
+
+func TestAllocDir_DetectContentType(t *testing.T) {
+	require := require.New(t)
+	imgPath := "input/image.png"
+	fileInfo, err := os.Stat(imgPath)
+	require.Nil(err)
+	res := detectContentType(fileInfo, imgPath)
+	require.Equal("image/png", res)
+
+	jsonPath := "input/test.json"
+	fileInfo, err = os.Stat(jsonPath)
+	require.Nil(err)
+	res = detectContentType(fileInfo, jsonPath)
+	require.Equal("application/json", res)
+}

client/allocdir/input/test.json

@@ -0,0 +1,3 @@
+{
+"test":"test"
+}