A user cannot add comments or description in a issue from his Mac, but he can from Windows.
I checked the JIRA Drag and Drop Attachment Plugin and it's active.
Daniela,
Sounds like a browser issue. What browser is he using on his Mac? Check and make sure the browser is supported.
Server - https://confluence.atlassian.com/adminjiraserver072/supported-platforms-828787550.html
Cloud - https://confluence.atlassian.com/cloud/supported-browsers-744721663.html
If it is I would start with clearing cache to ensure that he has the latest javascript for JIRA cached on his Mac.
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