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/magic/jpegxl_internal_test.go

@@ -1,12 +1,17 @@
 package magic
 
-import "testing"
+import (
+	"testing"
+
+	"sneak.berlin/go/pixa/internal/imagerequest"
+)
 
 // testMIMEJXL is the MIME type of JPEG XL.
 const testMIMEJXL = "image/jxl"
 
 // TestDetectFormatJPEGXL verifies that both JPEG XL signatures, that of a
-// bare codestream and that of the container, are detected as image/jxl.
+// bare codestream and that of the container, are detected as JPEG XL and
+// match the MIME type image/jxl.
 func TestDetectFormatJPEGXL(t *testing.T) {
 	t.Parallel()
 
@@ -25,25 +30,15 @@ func TestDetectFormatJPEGXL(t *testing.T) {
 			t.Parallel()
 
 			got, err := DetectFormat(tt.data)
-			if err != nil || got != MIMETypeJXL {
-				t.Errorf("DetectFormat() = %q, %v, want %q", got, err, testMIMEJXL)
+			if err != nil || got != imagerequest.FormatJXL {
+				t.Errorf("DetectFormat() = %q, %v, want %q",
+					got, err, imagerequest.FormatJXL)
 			}
 
-			err = ValidateMagicBytes(tt.data, testMIMEJXL)
+			_, err = ValidateMagicBytes(tt.data, testMIMEJXL)
 			if err != nil {
 				t.Errorf("ValidateMagicBytes(%q) error = %v", testMIMEJXL, err)
 			}
 		})
 	}
 }
-
-// TestJPEGXLMIMEType verifies that image/jxl maps to the format jxl.
-func TestJPEGXLMIMEType(t *testing.T) {
-	t.Parallel()
-
-	format, ok := MIMEToImageFormat(testMIMEJXL)
-	if format != FormatJXL || !ok {
-		t.Errorf("MIMEToImageFormat(%q) = %q, %v, want %q, true",
-			testMIMEJXL, format, ok, FormatJXL)
-	}
-}