JIRA does not have a native means to edit attachments on the fly in the browser. You can edit attachments and then re-upload them after changes have been made. But in those cases, you would have to save the file to your local filesystem first anyways in order to then make changes and upload it again.
Perhaps you would be interested in this plugin for JIRA: GoEdit - Direct Attachment Edit for JIRA
It appears to provide the kind of functionality I think you are looking for here.
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