An Active Storage `Blob` must be identified before it can be reliably
validated.  For direct uploads, a `Blob` is identified when it is
attached, rather than when it is created.

Before this commit, the sequence of events when attaching a `Blob` was:

1. Find the `Blob`.
2. Assign the `Blob` to an `Attachment`.
3. Save the owner record.
4. Save the `Attachment`.
5. Identify the `Blob`'s true `content_type` from its file.
6. Save the `Blob`.

This meant that the owner record's validations might not see the
`Blob`'s true `content_type`.

After this commit, the sequence of events will be:

1. Find the `Blob`.
2. Identify the `Blob`'s true `content_type` from its file.
3. Assign the `Blob` to an `Attachment`.
4. Save the owner record.
5. Save the `Attachment`.
6. Save the `Blob`.

Thus the `Blob`'s true `content_type` will be available when running the
owner record's validations.