mime: add mimetype for .jxl files

[akmittal]

Serve .jxl files with image/jxl mimetype
Fixes #70165

[gopherbot]

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from Amit Mittal:

Patch Set 2:

(1 comment)

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[gopherbot]

Message from Ian Lance Taylor:

Patch Set 2:

(1 comment)

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