diff --git a/DFReader.py b/DFReader.py
index 3e20ae367..480650378 100644
--- a/DFReader.py
+++ b/DFReader.py
@@ -1086,8 +1086,10 @@ def make_format_msgbuf(self, fmt):
 def DFReader_is_text_log(filename):
     '''return True if a file appears to be a valid text log'''
     with open(filename, 'r') as f:
-        ret = (f.read(8000).find('FMT,') != -1)
-
+        try:
+            ret = (f.read(8000).find('FMT,') != -1)
+        except UnicodeDecodeError:
+            ret = False
     return ret
 
 
diff --git a/tools/mavlogdump.py b/tools/mavlogdump.py
index f771790a1..9765b2466 100755
--- a/tools/mavlogdump.py
+++ b/tools/mavlogdump.py
@@ -1,8 +1,9 @@
 #!/usr/bin/env python
 
 '''
-example program that dumps a Mavlink log file. The log file is
-assumed to be in the format that qgroundcontrol uses, which consists
+Dumps out the contents a log file in the requsted format.
+The input log file may by a dataflash file in binary (.bin, .px4log)
+or text (.log) format, or a telemetry log (.tlog, .log) consisting of
 of a series of MAVLink packets, each with a 64 bit timestamp
 header. The timestamp is in microseconds since 1970 (unix epoch)
 '''
@@ -69,6 +70,7 @@
 import inspect
 
 from pymavlink import mavutil
+from pymavlink import DFReader
 
 
 if args.profile:
@@ -99,10 +101,9 @@
 if nottypes is not None:
     nottypes = nottypes.split(',')
 
-ext = os.path.splitext(filename)[1]
-isbin = ext in ['.bin', '.BIN', '.px4log']
-islog = ext in ['.log', '.LOG'] # NOTE: "islog" does not mean a tlog
-istlog = ext in ['.tlog', '.TLOG']
+isbin = isinstance(mlog,DFReader.DFReader_binary)
+islog = isinstance(mlog,DFReader.DFReader_text) # NOTE: "islog" does not mean a tlog
+istlog = isinstance(mlog,mavutil.mavmmaplog)
 
 # list of msgs to reduce in rate when --reduce is used
 reduction_msgs = ['NKF*', 'XKF*', 'IMU*', 'AHR2', 'BAR*', 'ATT', 'BAT*', 'CTUN', 'NTUN', 'GP*', 'IMT*', 'MAG*', 'PL', 'POS', 'POW*', 'RATE', 'RC*', 'RFND', 'UBX*', 'VIBE', 'NKQ*', 'MOT*', 'CTRL', 'FTS*', 'DSF', 'CST*', 'LOS*', 'UWB*']