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/channels/app/imaging/decode.go

@@ -16,6 +16,7 @@ import (
 	_ "github.com/oov/psd"
 	_ "golang.org/x/image/bmp"
 	_ "golang.org/x/image/tiff"
+	_ "golang.org/x/image/webp"
 )
 
 // DecoderOptions holds configuration options for an image decoder.