There is a plugin available for purchase on the Atlassian Marketplace that allows for checkout functionality with attachments. Confluence does not natively have attachment checkout. I believe this will meet your requirements for single user edits at a time on an attachment.
This really should be standard functionality, and there are long standing Confluence feature requests for this - please consider watch/vote/comment on the following issues:
@Chuck Minarik Excellentable spreadsheet for confluence provides collaborative editing. Now multiple users can edit the same spreadsheet without waiting for other users to complete their work. We are supporting up to 5 users to work in the same session and are working to support it up to 20 simultaneous users in the near future.
Two vulnerabilities have been published for Confluence Server and Data Center recently: March 20, 2019 CVE-2019-3395 / CVE-2019-3396 April 17, 2019 CVE-2019-3398 The goal of this article is...
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