If you're using WebUI, the upload providers support an accept attribute, where the value is a comma-separated list of accepted mime types.

https://doc.nuxeo.com/nxdoc/web-ui-upload-providers/

Here's an example:

    <nuxeo-dropzone role="widget"
                label="PDF file"
                name="content"
                accept="application/pdf"
                document="[[document]]"></nuxeo-dropzone>