Optional mimeType for ImageDecoder?

[jakearchibald]

Right now, you need to provide a mimeType for ImageDecoder, but createImageBitmap uses sniffing.

Could the ImageDecoder mimetype be optional and use sniffing if one isn't provided?

[jakearchibald]

Some more context: On squoosh.app we'd like to use ImageDecoder, particularly for animated file formats.

Right now <img> supports animated formats, even if the mime type is incorrect.

If we want to match the support of <img> where the file extension or mime type doesn't matter, but be able to grab particular frames from the animated format, we would need to implement our own sniffing, even though that code already exists in the browser.

[dalecurtis]

This is a duplicate of #169 -- please add your discussion there.

[jakearchibald]

Hah, sorry, I searched for "mimetype" but it doesn't appear in that issue