dms/testERP5WebWithDms: fix test uploading a png in a web page

use Image portal type to store image data. Using a Web Page never made
sense, it was working on python2 because the lax difference between
between text and bytes.

This test a svg contained in various portal types (Image, Web Page, )
with a png background image. The problem was with the background image.
This change does not affect the portal type containing the svg, only
the png background image.

bt5/erp5_dms/TestTemplateItem/portal_components/test.erp5.testERP5WebWithDms.py

@@ -923,12 +923,10 @@ return True
         url at the url of the image tag. ie:
          <image xlink:href="http://www.erp5.com/user-XXX-XXX"
     """
-    portal = self.portal
-    module = portal.getDefaultModule(portal_type=portal_type)
-    upload_file = self.makeFileUpload('user-TESTSVG-BACKGROUND-IMAGE.png')
-    background_image = module.newContent(portal_type=portal_type,
-                                    file=upload_file,
-                                    reference="NXD-BACKGROUND")
+    background_image = self.portal.image_module.newContent(
+      portal_type='Image',
+      file=self.makeFileUpload('user-TESTSVG-BACKGROUND-IMAGE.png'),
+      reference="NXD-BACKGROUND")
     background_image.publish()
     self.tic()