Using showcase application the Overview example uses both multiple="true" attribute with required="true" and requiredMessage attributes.
If you remove the multiple="true" and other multiple file upload attributes, and place a submit button on the form, the fileEntry uploads the file without any problem.
However, with the multiple="true" and buttons which are part of this attribute on the component, the fileName is missing and it fails validation.
Note that there are multiple Content-Dispostion: values submitted for each request and that one has an empty string or filename....for example...

. ------WebKitFormBoundary7Juu0QI8HITmC6FJ
. Content-Disposition: form-data; name="example-form:file-entry"; filename="Dill Pickle Rag.pdf"
. Content-Type: application/pdf
.
.
. ------WebKitFormBoundary7Juu0QI8HITmC6FJ
. Content-Disposition: form-data; name="example-form:file-entry"; filename=""
. Content-Type: application/octet-stream
.
....
this appears to work fine in all cases except when using multiple="true" on this particular application server.

The FileEntryUpload then does not execute the captureFormField method at al ( still debugging that at the moment).