make sure we have the image file for theme backgrounds

Fixes: https://launchpad.net/bugs/1197376

openlp/core/ui/themeform.py

@@ -38,7 +38,7 @@ from openlp.core.lib import UiStrings, Registry, translate
 from openlp.core.lib.theme import BackgroundType, BackgroundGradientType
 from openlp.core.lib.ui import critical_error_message_box
 from openlp.core.ui import ThemeLayoutForm
-from openlp.core.utils import get_images_filter
+from openlp.core.utils import get_images_filter, is_not_image_file
 from themewizard import Ui_ThemeWizard
 
 log = logging.getLogger(__name__)
@@ -178,7 +178,7 @@ class ThemeForm(QtGui.QWizard, Ui_ThemeWizard):
         """
         background_image = BackgroundType.to_string(BackgroundType.Image)
         if self.page(self.currentId()) == self.backgroundPage and \
-                self.theme.background_type == background_image and not self.imageFileEdit.text():
+                self.theme.background_type == background_image and is_not_image_file(self.imageFileEdit.text()):
             QtGui.QMessageBox.critical(self, translate('OpenLP.ThemeWizard', 'Background Image Empty'),
                 translate('OpenLP.ThemeWizard', 'You have not selected a '
                     'background image. Please select one before continuing.'))

openlp/core/utils/__init__.py

@@ -246,6 +246,23 @@ def get_images_filter():
     return IMAGES_FILTER
 
 
+def is_not_image_file(file_name):
+    """
+    Validate that the file is not an image file.
+
+    ``file_name``
+        File name to be checked.
+    """
+    if file_name.isEmpty():
+        return True
+    else:
+        formats = [fmt.lower() for fmt in QtGui.QImageReader.supportedImageFormats()]
+        file_part, file_extension = os.path.splitext(unicode(file_name))
+        if file_extension[1:].lower() in formats and os.path.exists(file_name):
+            return False
+        return True
+
+
 def split_filename(path):
     """
     Return a list of the parts in a given path.