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Upon further investigation, it seems that the problem is not with wp_attachment_is_image at all, but because the image size (width and height) are not being added to the attachment metadata when the SVG is uploaded. That's because the function to calculate the image used is the php function getimagesize(), which does not return an image size for SVG. I found an answer on stackoverflow about the getimagesize function and about how svgs behave. See it here.

Upon further investigation, it seems that the problem is not with wp_attachment_is_image at all, but because the image size (width and height) are not being added to the attachment metadata when the SVG is uploaded. That's because the function to calculate the image used is the php function getimagesize(), which does not return an image size for SVG. I found an answer on stackoverflow about the getimagesize function and about how svgs behave. See it here.

UPDATE

Upon further investigation, it seems that the problem is not with wp_attachment_is_image at all, but because the image size (width and height) are not being added to the attachment metadata when the SVG is uploaded. That's because the function to calculate the image used is the php function getimagesize(), which does not return an image size for SVG. I found an answer on stackoverflow about the getimagesize function and about how svgs behave. See it here.

I am not sure why the last if statement returns false for svgs though. Even if I don't add the svg extension to the array $image_exts, the first condition should return true, shouldn't it?

if ( 'image/' == substr($post->post_mime_type, 0, 6)

That checks if 'image/' is enqual to the first six character in the mime type, which for svg is image/svg+xml (first six is "image/").