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/httpfetcher/mock.go

@@ -66,23 +66,29 @@ func (m *MockFetcher) Fetch(
 	}, nil
 }
 
+// The content types the mock fetcher gives files in no format pixa reads.
+const (
+	contentTypeSVG         = "image/svg+xml"
+	contentTypeOctetStream = "application/octet-stream"
+)
+
 // detectContentTypeFromPath returns the MIME type based on file extension.
 func detectContentTypeFromPath(path string) string {
 	path = strings.ToLower(path)
 
 	switch {
 	case strings.HasSuffix(path, ".jpg"), strings.HasSuffix(path, ".jpeg"):
-		return contentTypeJPEG
+		return imagerequest.FormatJPEG.MIMEType()
 	case strings.HasSuffix(path, ".png"):
-		return contentTypePNG
+		return imagerequest.FormatPNG.MIMEType()
 	case strings.HasSuffix(path, ".gif"):
-		return contentTypeGIF
+		return imagerequest.FormatGIF.MIMEType()
 	case strings.HasSuffix(path, ".webp"):
-		return contentTypeWebP
+		return imagerequest.FormatWebP.MIMEType()
 	case strings.HasSuffix(path, ".avif"):
-		return contentTypeAVIF
+		return imagerequest.FormatAVIF.MIMEType()
 	case strings.HasSuffix(path, ".jxl"):
-		return contentTypeJXL
+		return imagerequest.FormatJXL.MIMEType()
 	case strings.HasSuffix(path, ".svg"):
 		return contentTypeSVG
 	default: