Add WebP to MediaFormats #26628 (#26805)

Changes the default content disposition for WebP attachments
from *download* to *inline*.

Add WebP codec support side-effect to decode.go. To prevent future
issues caused by possible changes in emoji.go.

server/platform/shared/web/files.go

@@ -27,6 +27,7 @@ var MediaContentTypes = [...]string{
 	"image/bmp",
 	"image/gif",
 	"image/tiff",
+	"image/webp",
 	"video/avi",
 	"video/mpeg",
 	"video/mp4",