Tooltips can be attached to each one of FileUpload buttons in advanced mode using PrimeFaces Selectors. Moreover, you can use plain html browser native titles as well.
<div class="card">
<h:form id="formId">
<p:fileUpload id="uploader1" mode="advanced" styleClass="block mb-5"/>
<p:outputPanel id="tooltipsPanel">
<p:tooltip id="uploaderChooseFileBeforeUploadToolTip"
widgetVar="chooseWV" position="top"
for="@(#formId\:uploader1 > .ui-fileupload-buttonbar > .ui-fileupload-choose)"
value="Choose button tooltip"/>
<p:tooltip id="uploaderUploadButtonToolTip"
position="right"
for="@(#formId\:uploader1 > .ui-fileupload-buttonbar > .ui-fileupload-upload)"
value="Upload button tooltip" showEffect="clip"
hideEffect="explode"/>
<p:tooltip id="uploaderCancelButtonToolTip"
position="bottom"
for="@(#formId\:uploader1 > .ui-fileupload-buttonbar > .ui-fileupload-cancel)"
value="Cancel button tooltip"/>
</p:outputPanel>
<p:fileUpload id="uploader2" mode="advanced" styleClass="block mb-5"
chooseButtonTitle="Choose button tooltip"
uploadButtonTitle="Upload button tooltip"
cancelButtonTitle="Cancel button tooltip"/>
<p:fileUpload id="uploader3" mode="simple" skinSimple="true" styleClass="block mb-5"
title="my custom title"/>
<p:fileUpload id="uploader4" mode="simple" styleClass="block mb-5"
title="my custom title"/>
</h:form>
</div>