Fix saving of non-RGB thumbnails as PNG

The PNG format does not support non-RGB color spaces, which makes pillow
raise an exception when trying to save e.g. a CMYK image in PNG format.

Converting to RGB is recommended on the upstream issue, and as CMYK is a
print format transparency does not need to be considered.

Closes: element-hq#10741

synapse/media/thumbnailer.py

@@ -206,7 +206,7 @@ def crop(self, width: int, height: int, output_type: str) -> BytesIO:
     def _encode_image(self, output_image: Image.Image, output_type: str) -> BytesIO:
         output_bytes_io = BytesIO()
         fmt = self.FORMATS[output_type]
-        if fmt == "JPEG":
+        if fmt == "JPEG" or fmt == "PNG" and output_image.mode == "CMYK":
             output_image = output_image.convert("RGB")
         output_image.save(output_bytes_io, fmt, quality=80)
         return output_bytes_io