When attempting to save a draw.io diagram in JIRA Cloud the status bar displays the message "Error saving file. Insufficient access rights"
You can customized the default permissions at some level and that has removed the ability for the current user to write or change a diagram
In JIRA Cloud there is a permissions helper under:
(replace the 2 placeholders to your settings)
There you can enter the user, the issue and the permission to check (create attachments if a new diagram or create and delete own attachments if existing). When you press submit it will tell you why the user cannot perform the required action.
Note that there is no operation to update an existing attachment in JIRA Cloud, a delete followed by a create are required.