refactor thumbnails

Signed-off-by: Christian Richter <crichter@owncloud.com>

extensions/thumbnails/pkg/thumbnail/encoding_test.go

@@ -0,0 +1,22 @@
+package thumbnail
+
+import "testing"
+
+func TestEncoderForType(t *testing.T) {
+	table := map[string]Encoder{
+		"jpg":     JpegEncoder{},
+		"JPG":     JpegEncoder{},
+		"jpeg":    JpegEncoder{},
+		"JPEG":    JpegEncoder{},
+		"png":     PngEncoder{},
+		"PNG":     PngEncoder{},
+		"invalid": nil,
+	}
+
+	for k, v := range table {
+		e, _ := EncoderForType(k)
+		if e != v {
+			t.Fail()
+		}
+	}
+}