When configuring the “valid file extensions” in the knime multiple file upload widget to have more than one file extension (as shown below), only the first extension type can be uploaded and the rest will be greyed out.
However on the main upload screen it seems that knime correctly recognizes that three potential file types can be uploaded as seen on the message circled:
i have looked through the forums and it seems that a similar issue existed back in AP 4.5.2 almost 4 years ago, however i am running AP 5.8.1, hopefully there will be a fix soon.