SaveImage command saves images without a file extension

[HubertDuck]

twitterで「画像を保存する」のジェスチャーを使うと、
ファイルに拡張子が付かない。
twitter以外のサイトでは拡張子が付きます。

[Robbendebiene]

It is hard to decide whether this is a bug or not. The Save Image command basically saves the file as it is. Since the image does not include any file extension itself it is saved without it.

What makes it feel like a bug is that Firefox appends/generates a file extension when downloading it via right click "Save Image".

I would love to mimic this, but there is no way to determine the MIME type of the image before downloading it. It would probably be possible if this would be implemented: https://bugzilla.mozilla.org/show_bug.cgi?id=1245652
One could also argue that Firefox should do this automatically when using the web extensions downloads API without specifying a file name.

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reference: https://stackoverflow.com/questions/31314035/htmlimageelement-how-to-get-image-type-if-its-src-doesnt-have-extension