file input: Show accepted media types.

plone/formwidget/namedfile/file_input.pt

@@ -140,6 +140,14 @@
              maxlength view/maxlength;
            "
     />
+    <div class="form-text"
+         tal:condition="view/accept"
+         i18n:translate="namedfile_accepted_types"
+    >
+      Allowed types:
+      <tal:i18n i18n:name="accepted_types">${view/accept}</tal:i18n>.
+    </div>
+
     <script type="text/javascript"
             tal:condition="python:allow_nochange and action != 'replace'"
             tal:content="string:document.getElementById('${view/id}-input').disabled=true;"

plone/formwidget/namedfile/image_input.pt

@@ -145,6 +145,14 @@
              maxlength view/maxlength;
            "
     />
+    <div class="form-text"
+         tal:condition="view/accept"
+         i18n:translate="namedfile_accepted_types"
+    >
+      Allowed types:
+      <tal:i18n i18n:name="accepted_types">${view/accept}</tal:i18n>.
+    </div>
+
     <script type="text/javascript"
             tal:condition="python:allow_nochange and action != 'replace'"
             tal:content="string:document.getElementById('${view/id}-input').disabled=true;"

plone/formwidget/namedfile/widget.rst

@@ -285,8 +285,10 @@ At first, there is no value, so the behaviour is much like before::
   >>> print(image_widget.render())
   <span id="widget.id.image" class="named-image-widget required namedimage-field" >
       <input accept="image/*" type="file" id="widget.id.image-input" name="widget.name.image" />
+      <div class="form-text" >Allowed types: image/*.</div>
   </span>
 
+
 However, if we now set a value, we will have the option of keeping it,
 or changing it.  The filename can handle unicode and international
 characters::
@@ -440,12 +442,14 @@ At first, there is no value, so the behaviour is much like before::
   >>> print(file_widget_constrained.render())
   <span id="widget.id.file" class="named-file-widget required namedfile-field" >
       <input accept="audio/mp3, audio/flac, .wav" type="file" id="widget.id.file-input" name="widget.name.file" />
+      <div class="form-text" >Allowed types: audio/mp3, audio/flac, .wav.</div>
   </span>
 
   >>> image_widget_constrained.update()
   >>> print(image_widget_constrained.render())
   <span id="widget.id.image" class="named-image-widget required namedimage-field" >
       <input accept="image/webp, image/png, .jpg" type="file" id="widget.id.image-input" name="widget.name.image" />
+      <div class="form-text" >Allowed types: image/webp, image/png, .jpg.</div>
   </span>