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/imagerequest/imagerequest.go

@@ -1,7 +1,8 @@
 // Package imagerequest holds the image request type, its parts, its checks,
 // the parsing of the /v1/image/ path, the variant key, the source key and the
-// source's upstream URL. It imports no other package of this repo, so every
-// package that handles an image request can import it.
+// source's upstream URL. It also holds the image format type and the MIME type
+// of each format. It imports no other package of this repo, so every package
+// that handles an image request can import it.
 package imagerequest
 
 import (
@@ -12,10 +13,11 @@ import (
 // ErrInvalidFitMode is returned when an invalid fit mode is provided.
 var ErrInvalidFitMode = errors.New("invalid fit mode")
 
-// Format represents supported output image formats.
+// Format is an image format. pixa reads and writes the formats from JPEG to
+// GIF below. In a request, orig and auto stand for one of them.
 type Format string
 
-// Supported image output formats.
+// Supported image formats.
 const (
 	FormatOriginal Format = "orig"
 	FormatJPEG     Format = "jpeg"
@@ -31,6 +33,30 @@ const (
 	FormatAuto Format = "auto"
 )
 
+// MIMEType returns the MIME type of the format, such as image/jpeg for
+// FormatJPEG. orig and auto stand for another format, so they have none:
+// MIMEType returns "" for them.
+func (f Format) MIMEType() string {
+	switch f {
+	case FormatJPEG:
+		return "image/jpeg"
+	case FormatPNG:
+		return "image/png"
+	case FormatWebP:
+		return "image/webp"
+	case FormatAVIF:
+		return "image/avif"
+	case FormatJXL:
+		return "image/jxl"
+	case FormatGIF:
+		return "image/gif"
+	case FormatOriginal, FormatAuto:
+		return ""
+	default:
+		return ""
+	}
+}
+
 // Size represents requested image dimensions
 type Size struct {
 	Width  int