This pages illustrates what you can expect when a limit on the allowed file extensions for the files that may be attached to the Jira issues has been configured for a project.
Assuming the following setting:
When an user try to attach a file having an extension that is not included in the list of the allowed extensions, an alert appears and the file is not attached to the issue.
On the issue Create and Edit screens the alert displays as follows:
On the issue View screen the alert displays as follows: