Update test to use non-AVIF format since now supported in 6.5-alpha

[westonruter]

Now that AVIF support just landed in WP trunk via Core-51228, we need to use a different image format to check for an invalid MIME type. This fixes a unit test failure on 6.5-alpha.

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Props: westonruter, adamsilverstein, joemcgill.

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Co-authored-by: westonruter <westonruter@git.wordpress.org>
Co-authored-by: adamsilverstein <adamsilverstein@git.wordpress.org>
Co-authored-by: joemcgill <joemcgill@git.wordpress.org>

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[adamsilverstein]

Looks good! I've never seen a .pcx file (but that is the point I guess)

[westonruter]

Looks good! I've never seen a .pcx file (but that is the point I guess)

I used the PCX format a lot with PC Paintbrush as a kid 😄

[joemcgill]

This works.