Using Internet Explorer, if you try to attach a file with no content (0 byte size), the file appears to be uploaded, but it is not. No warning or error message is displayed. For other browsers, when you select a 0 byte file, an error message is displayed and you cannot upload the file.
This is a limitation of Internet Explorer. It is not possible to detect 0 byte files because obtaining the size of a file on a local computer using the HTML <input> element is not supported.
IBM Connections Mail on Internet Explorer
Resolving the problem
To avoid this issue, use a different supported browser. This issue has been reported to IBM Connections Mail development.