Make ImageFormField render with accept="image/*"' HTML attribute

Since Django 2.1, the default forms.ImageField has rendered with
accept="image/*".  Copy the code here to do the same.

sorl/thumbnail/fields.py

@@ -2,6 +2,7 @@
 from django.db.models import Q
 from django import forms
 from django.utils.translation import gettext_lazy as _
+from django.forms.widgets import FileInput
 
 from sorl.thumbnail import default
 
@@ -68,3 +69,9 @@ def to_python(self, data):
             f.seek(0)
 
         return f
+
+    def widget_attrs(self, widget):
+        attrs = super().widget_attrs(widget)
+        if isinstance(widget, FileInput) and 'accept' not in widget.attrs:
+            attrs.setdefault('accept', 'image/*')
+        return attrs