Fixed #27007 -- Handled non-UTF-8 bytes objects for text/* attachments.

The fallback logic which allows non-UTF-8 encoded files to be passed to
attach_file() even when a `text/*` mime type has been specified is
moved to attach(). Both functions now fall back to a content type of
`application/octet-stream`.

A side effect is that a file's content is decoded in memory instead of
opening it in text mode and reading it into a string.

Some mimetype-related logic in _create_attachment() has become
obsolete as the code moved from attach_file() to attach() already
handles this.

docs/releases/1.11.txt

@@ -162,6 +162,10 @@ Email
 * Added the :setting:`EMAIL_USE_LOCALTIME` setting to allow sending SMTP date
   headers in the local time zone rather than in UTC.
 
+* ``EmailMessage.attach()`` and ``attach_file()`` now fall back to MIME type
+  ``application/octet-stream`` when binary content that can't be decoded as
+  UTF-8 is specified for a ``text/*`` attachment.
+
 File Storage
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