Fix SVG upload failure caused by getimagesize() validation

Jonathan Danaherrequested to merge
issue/filepond-3601096:1.0.x into 1.0.x

getimagesize() does not support SVG files, causing uploads to fail with "not a valid image" when SVG is an allowed type. Skip raster image validation and dimension capture for image/svg+xml, and add an early return in validateImageContent() that checks SVGs against allowed MIME types only.

Fixes #3601096.

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