Add skel for preview generator for ggs

Co-authored-by: Ralf Haferkamp <rhafer@owncloud.com>

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

services/thumbnails/pkg/thumbnail/encoding.go

@@ -15,6 +15,7 @@ const (
 	typeJpg  = "jpg"
 	typeJpeg = "jpeg"
 	typeGif  = "gif"
+	typeGgs  = "ggs"
 )
 
 var (
@@ -119,6 +120,8 @@ func GetExtForMime(fileType string) string {
 	switch ext {
 	case typeJpg, typeJpeg, typePng, typeGif:
 		return ext
+	case "application/vnd.geogebra.slides":
+		return typeGgs
 	default:
 		return ""
 	}

services/thumbnails/pkg/thumbnail/generator.go

@@ -71,6 +71,14 @@ func (g GifGenerator) imageToPaletted(img image.Image, p color.Palette) *image.P
 	return pm
 }
 
+// GgsGenerator is used to create a web friendly version of the provided ggs image.
+type GgsGenerator struct{}
+
+func (g GgsGenerator) Generate(size image.Rectangle, img interface{}, processor Processor) (interface{}, error) {
+	// TODO: write zip extractor, get image from zip, process image, return image
+	return nil, nil
+}
+
 // GeneratorForType returns the generator for a given file type
 // or nil if the type is not supported.
 func GeneratorForType(fileType string) (Generator, error) {
@@ -79,6 +87,8 @@ func GeneratorForType(fileType string) (Generator, error) {
 		return SimpleGenerator{}, nil
 	case typeGif:
 		return GifGenerator{}, nil
+	case typeGgs:
+		return GgsGenerator{}, nil
 	default:
 		return nil, ErrNoEncoderForType
 	}