If you try to attach a file in transition screen when the destination workflow step has property jira.issue.editable=false, then file won't be actually attached. Although GUI won't show any error message, in the log file you will find something like this:

http-nio-8080-exec-16 WARN admin 770x585x1 1l07jej 0:0:0:0:0:0:0:1 /secure/CommentAssignIssue.jspa [c.a.j.issue.fields.AttachmentSystemField] There were errors while converting temporary attachments: AttachmentError

Unknown macro: {logMessage='An error occurred}

Affected versions seems to be JIRA 6.4.x to 7.0.0. Version JIRA 6.3.15 works fine.