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

docs/database/attachments.md

@@ -93,7 +93,9 @@ Originals in JPEG, PNG, GIF and WebP can be thumbnailed. The thumbnailer cannot
 
 `attach.Store` turns an upload into a `system_files` row. It takes an `attach.Upload` (the client's file name, the body and the public flag) and an `attach.Limits`, writes the bytes under the WinterCMS partition key and inserts an unattached row whose `sort_order` equals its id, as WinterCMS's Sortable trait does. The caller attaches the row later by setting `Field`, `AttachmentType` and `AttachmentID`, usually when the record's form is saved. When the row insert fails, the blob is deleted again.
 
-The client file name never reaches a blob key. The disk name is 22 random hexadecimal characters plus the lower-cased extension, and the extension must be one to ten letters or digits from the allowed list: `Limits.Extensions`, or, when that is empty, `attach.DefaultImageExtensions` in image mode (jpg, jpeg, png, gif and webp) and `attach.DefaultFileExtensions` otherwise. The file list is WinterCMS's default list without the types that can carry script: svg, js, map, css, less, scss, swf and xml. A refused extension returns `attach.ErrFileType`.
+The client file name never reaches a blob key. The disk name is 22 random hexadecimal characters plus an extension, and the client's lower-cased extension must be one to ten letters or digits from the allowed list: `Limits.Extensions`, or, when that is empty, `attach.DefaultImageExtensions` in image mode (jpg, jpeg, png, gif and webp) and `attach.DefaultFileExtensions` otherwise. The file list is WinterCMS's default list without the types that can carry script: svg, js, map, css, less, scss, swf and xml. A refused extension returns `attach.ErrFileType`.
+
+The disk name's extension follows the content, not the client's name, because a web server or CDN in front of the bucket picks the Content-Type from the extension. `attach.DiskExtension` derives it from the sniffed type and the client's extension. For a specific type such as PNG, JPEG, PDF, MP3 or WebM, the client's extension is kept when it names that type, else the type's usual extension is used: a PNG uploaded as `x.html` (with `html` allowed) is stored as `<hex>.png`, and its URL ends in `.png`. For a generic sniff (plain text, HTML, XML, a zip container or unknown bytes) the client's extension is kept, so `report.docx` stays `.docx` and an HTML body uploaded as `note.txt` stays `.txt`, unless the extension is one a browser renders or runs (html, svg, xml, js, css, php and the like), which becomes `.txt` for text and `.bin` otherwise. `file_name` always keeps the client's name, so downloads still carry it.
 
 The content type is sniffed from the first bytes of the body, never taken from the client. `Limits.MIMETypes` narrows it further: an entry with a slash is a MIME pattern such as `image/png` or `image/*`, an entry without one is an extension, and a file that matches no entry returns `attach.ErrMIMEType`. `Limits.MaxBytes` is enforced while the body streams into the bucket; a body one byte over the limit returns `attach.ErrTooLarge` and leaves no blob and no row behind.