Fix project import from remote to import from S3

What does this MR do and why?

Currently, Import::GitlabProjects::CreateProjectFromRemoteFileService validates:

1. The file size based on the Content-Length header, which S3 doesn't returb on head requests;
2. The file content type, which S3 use application/x-tar instead of application/gzip for a tar.gz;

Related to: #337230 (closed)

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