Allow showing png and jpg contents as invocation outputs

lib/galaxy/datatypes/data.py

@@ -482,6 +482,8 @@ class Data(object):
         If the data cannot reasonably be displayed, just indicate this and do
         not throw an exception.
         """
+        if self.file_ext in {'png', 'jpg'}:
+            return self.handle_dataset_as_image(dataset_instance)
         if self.is_binary:
             result = "*cannot display binary content*\n"
         else:

lib/galaxy/datatypes/images.py

@@ -1,6 +1,7 @@
 """
 Image classes
 """
+import base64
 import logging
 import zipfile
 
@@ -46,6 +47,13 @@ class Image(data.Data):
         """Determine if the file is in this format"""
         return check_image_type(filename, self.image_formats)
 
+    def handle_dataset_as_image(self, hda):
+        dataset = hda.dataset
+        name = hda.name or ''
+        with open(dataset.file_name, "rb") as f:
+            base64_image_data = base64.b64encode(f.read()).decode("utf-8")
+        return "![%s](data:image/%s;base64,%s)" % (name, self.file_ext, base64_image_data)
+
 
 class Jpg(Image):
     edam_format = "format_3579"