Thumbnails API compatibility with oc10

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As oCIS developers we want the thumbnails API to be compatible with the OC10 API so that setups with oc10 backend and oCIS web frontend can easily transition to oCIS backend.

Acceptance criteria

• The APIs bahave the same (can be verified using the acceptance tests)

[exalate-issue-sync]

David Christofas commented: I looked through the expected failures and many of them will be solve with this PR: #1898

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David Christofas commented: Most of the failures in the test suite are from thumbnails for .txt files which we don't yet support in ocis.

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David Christofas commented: Thumbnail support for .txt files was implemented here.
#1988

Now most of the remaining failing tests are about not matching resolutions. This is because we don't allow freely chosen resolutions but instead map the requested resolution to the closest matching pre configured one. That way it's not possible to spam the storage with thumbnail files.