fix error MediaType when get dup manifest from db

[fourierrr]

What this PR does / why we need it: When re-converting overlaybd image, the mediaType of the manifest read from the database should be "application/vnd.docker.distribution.manifest.v2+json", but it is incorrectly set to "application/vnd.docker.image.rootfs.diff.tar".

note: This issue only occurs in the case of converting multi-architecture images, as only during this conversion is a new index manifest uploaded. In the case of converting single-architecture images, the manifest is not uploaded, and the process ends directly.

Please check the following list:

• [ ] Does the affected code have corresponding tests, e.g. unit test, E2E test? no
• [ ] Does this change require a documentation update? no
• [ ] Does this introduce breaking changes that would require an announcement or bumping the major version? no
• [ ] Do all new files have an appropriate license header? no

[WaberZhuang]

Thanks for the PR, please add your signature to the commit using git commit -s ... to pass the project checks.