Would be better to use python-magic rather than the Python mimetypes module, as the mimetypes module relies on file extensions and log files use the PID for the file extension.

we could try something like:
--- a/bitbake/lib/toaster/toastergui/views.py
+++ b/bitbake/lib/toaster/toastergui/views.py
@@ -67,7 +67,10 @@ class MimeTypeFinder(object):
         guess = mimetypes.guess_type(path, self._strict)
         guessed_type = guess[0]
         if guessed_type == None:
-            guessed_type = 'application/octet-stream'
+            if "log.do_" in os.path.basename(path):
+                guessed_type = 'text/plain'
+            else:
+                guessed_type = 'application/octet-stream'
         return guessed_type


since we are already providing a base guess at least it would keep any special cases in one place.