Transform a JPEG uploaded as .jfif with the ImageMagick transformer

[robzolkos]

Fixes #84

Why

Rails names a web image's variant format after the upload's extension when Marcel maps that extension to the blob's content type. So an app using the ImageMagick transformer gets format jfif for a JPEG uploaded as photo.jfif. The same goes for .jif and .jfi, in any letter case.

Since #82, the transform saves straight to the output's scratch path, which has no extension, so image_processing names the output format with a prefix: jfif:/…/output-0. ImageMagick has no coder called JFIF. It reads the whole string as a filename, fails to open it, and the operation answers unreadable, a permanent failure, for a perfectly good JPEG. Stock Rails doesn't hit this, because it lets image_processing name a tempfile with the extension.

What changes

• The ImageMagick toolchain stages its output on a sibling named with the format's extension (Output#path(extension:)) and adopts it into place, as it did before Require image_processing 2.2.0 and drop the transform rename #82. ImageMagick picks the coder from an extension it knows, and keeps the source's format for one it doesn't. That's the same thing stock Rails gets.
• The Vips toolchain keeps Require image_processing 2.2.0 and drop the transform rename #82's direct write and its saved rename. Transforming#perform names the save path through a new encoded_path hook: the default is the scratch path, and Transformers::Image::Magick overrides it.
• Docs and changelog: the Output#path doc and comment now mention this second use of the extension sibling, and there's a changelog entry under ActiveStorage::HotCell::Server / Fixed.

An explicit svgz variant, which failed the same way, now behaves as it does under stock Rails too.

Tests

magick_transform_image_test.rb gets #84's reproduction for jfif, jif and jfi: a JPEG transformed to each format comes back as a resized JPEG.

• These are committed on their own first, in 54e9109, and fail on that commit with unable to open image 'jfif:…'.
• They pass with the fix.

bundle exec rake passes locally: every suite including test:activestorage, plus rubocop. rake docs:check is clean.

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[flavorjones]

Going to decline to merge in favor of hotcell#103 which modifies the format on the client side. I think that's cleaner and also improves the VIPS side of the house.