[ui][widgets][CodeMirrorFileWidget] Decode content read if bytes

If the data returned by the read method of the
object passed is byte, decode it. Otherwise
the TextArea will just show it as is.

beat/web/ui/widgets.py

@@ -346,6 +346,8 @@ class CodeMirrorFileWidget(CodeMirrorTextarea):
             contents = value
         elif hasattr(value, "read"):
             contents = value.read()
+            if type(contents) == bytes:
+                contents = contents.decode("utf-8")
         else:
             contents = ""
         return mark_safe(super().render(name, contents, attrs, renderer))  # nosec