fix(lagoon/attach): store non-inline uploads with Content-Disposition attachment (quick-261028-rqe)

- attach.IsInlineContentType: the shared inline raster list (JPEG, PNG,
  GIF, WebP, AVIF), parameters and case ignored, false for empty or
  unparsable values
- writeBlob stores Content-Disposition: attachment for every other type,
  so a public bucket or CDN makes the browser download HTML, SVG, PDF or
  plain text; content_type and thumbnails are unchanged
- driver tests on memblob and fileblob, Postgres-backed Store assertion,
  lagoon README and the storage and attachments pages

modules/lagoon/attach/inline.go

@@ -0,0 +1,44 @@
+package attach
+
+import (
+	"mime"
+	"strings"
+
+	"gocloud.dev/blob"
+)
+
+// inlineContentTypes are the raster image types a browser may show inline
+// from an upload.
+var inlineContentTypes = map[string]bool{
+	"image/avif": true,
+	"image/gif":  true,
+	"image/jpeg": true,
+	"image/png":  true,
+	"image/webp": true,
+}
+
+// attachmentDisposition is the Content-Disposition stored with a non-inline
+// upload. It is the bare value with no filename parameter: the client's file
+// name never reaches blob metadata or a response header.
+const attachmentDisposition = "attachment"
+
+// IsInlineContentType reports whether contentType, with parameters and case
+// ignored, is a raster image a browser may show inline from an upload (JPEG,
+// PNG, GIF, WebP, AVIF). An empty or unparsable value is not. Store stores
+// every other type, SVG, PDF, HTML and plain text included, with
+// Content-Disposition: attachment, and the surf uploads route serves it as a
+// download.
+func IsInlineContentType(contentType string) bool {
+	mediaType, _, err := mime.ParseMediaType(contentType)
+	return err == nil && inlineContentTypes[strings.ToLower(mediaType)]
+}
+
+// uploadWriterOptions are the blob writer options of an upload: the content
+// type, plus the attachment disposition when the type is not inline.
+func uploadWriterOptions(contentType string) *blob.WriterOptions {
+	opts := &blob.WriterOptions{ContentType: contentType}
+	if !IsInlineContentType(contentType) {
+		opts.ContentDisposition = attachmentDisposition
+	}
+	return opts
+}