Detect a fetched image's format from its bytes once (closes #319)

magic.DetectFormat returns an imagerequest.Format. ValidateMagicBytes
returns the format it checked against the upstream Content-Type. The
cache reads a cached source's format from its file's first bytes, as it
reads a variant's. The proxy hands the format to Process. Process
refuses a format that is not a source format, and input libvips would
load with another loader, such as an SVG passing as AVIF.
Format.MIMEType declares each MIME string once, and the fetcher, the
cache's variant read and the Accept matching use it. magic.MIMEType,
magic.ImageFormat, MIMEToImageFormat, magic's SVG detection,
detectFormat, formatFromString, FormatToMIME and Result.InputFormat are
gone.

Model: opus-5-5

internal/handlers/format_auto.go

@@ -65,20 +65,20 @@ func formatForAccept(accept string) (imagerequest.Format, error) {
 		return imagerequest.FormatJPEG, nil
 	}
 
-	if qualities["image/jxl"] > 0 {
+	if qualities[imagerequest.FormatJXL.MIMEType()] > 0 {
 		return imagerequest.FormatJXL, nil
 	}
 
-	if qualities["image/avif"] > 0 {
+	if qualities[imagerequest.FormatAVIF.MIMEType()] > 0 {
 		return imagerequest.FormatAVIF, nil
 	}
 
-	if qualities["image/webp"] > 0 {
+	if qualities[imagerequest.FormatWebP.MIMEType()] > 0 {
 		return imagerequest.FormatWebP, nil
 	}
 
 	// For JPEG, the most specific entry the header has decides
-	quality, named := qualities["image/jpeg"]
+	quality, named := qualities[imagerequest.FormatJPEG.MIMEType()]
 	if !named {
 		quality, named = qualities["image/*"]
 	}

internal/handlers/unsupported_source_internal_test.go

@@ -16,8 +16,8 @@ import (
 // TestUnsupportedSourceFormatAnswers415 checks that respondImageError, which
 // answers the errors of both image routes, answers 415 with a clear error when
 // the upstream answers with a content type pixa does not accept, such as SVG,
-// and when orig is asked of a source in a format the processor cannot name,
-// wrapped as the service wraps them.
+// and when libvips does not read a source as the format its first bytes
+// show, wrapped as the service wraps them.
 func TestUnsupportedSourceFormatAnswers415(t *testing.T) {
 	t.Parallel()
 
@@ -29,7 +29,7 @@ func TestUnsupportedSourceFormatAnswers415(t *testing.T) {
 	for _, err := range []error{
 		fmt.Errorf("upstream fetch failed: %w: image/svg+xml",
 			httpfetcher.ErrInvalidContentType),
-		fmt.Errorf("image processing failed: %w: unknown",
+		fmt.Errorf("image processing failed: %w: libvips does not read the input as avif",
 			imageprocessor.ErrUnsupportedInputFormat),
 	} {
 		rec := httptest.NewRecorder()