fix(19.3-03): name stored uploads after their sniffed content through attach.DiskExtension

- DiskExtension: a fixed table of specific sniffed types keeps a matching client
  extension or uses the canonical one; generic sniffs keep the client extension
  unless it is script-capable (then txt or bin); never asks package mime
- Store names the blob with DiskExtension(sniffed, ext); file_name, the allow-list
  and the stored content_type are unchanged
- TestStoreDiskExtensionSmoke: a PNG uploaded as x.html is stored and served as .png,
  report.docx keeps .docx, an HTML body named note.txt keeps .txt
- attachments docs on disk names, storage deploy note on nosniff and allow-lists,
  lagoon README

modules/lagoon/attach/store.go

@@ -95,7 +95,9 @@ type Limits struct {
 // checked against the sniffed type, or the extension's registered type when
 // the sniff only says application/octet-stream (else ErrMIMEType). The body
 // is then streamed into bucket at BlobKey of a server-generated disk name (22
-// random lowercase hex characters, a dot and the extension); a body longer
+// random lowercase hex characters, a dot and the extension DiskExtension
+// derives from the sniffed type and the client extension, so a PNG named
+// x.html is stored as .png; file_name keeps the client's name); a body longer
 // than Limits.MaxBytes aborts the write, deletes the key and returns
 // ErrTooLarge. Finally it inserts the row with empty attachment columns,
 // is_public from Upload.Public, the byte size and the content type, and sets
@@ -138,7 +140,8 @@ func Store(ctx context.Context, db *gorm.DB, bucket *blob.Bucket, in Upload, lim
 	if lim.Image && !IsAllowedImage(head) {
 		return nil, ErrNotImage
 	}
-	contentType := baseMediaType(http.DetectContentType(head))
+	sniffed := baseMediaType(http.DetectContentType(head))
+	contentType := sniffed
 	if contentType == "application/octet-stream" {
 		if byExt := baseMediaType(mime.TypeByExtension("." + ext)); byExt != "" {
 			contentType = byExt
@@ -148,7 +151,7 @@ func Store(ctx context.Context, db *gorm.DB, bucket *blob.Bucket, in Upload, lim
 		return nil, fmt.Errorf("%w: %s", ErrMIMEType, contentType)
 	}
 
-	diskName, err := newDiskName(ext)
+	diskName, err := newDiskName(DiskExtension(sniffed, ext))
 	if err != nil {
 		return nil, err
 	}