DFReader: is_text_log should return false if binary

In mavlogdump, use type of mlog not file ext to determine type to allow .log to be either text or tlog
Update description text in mavlogdump file header

DFReader.py

@@ -1085,8 +1085,10 @@ 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
 
 

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)
 '''
@@ -99,10 +100,10 @@
 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']
+mlog_type = str(type(mlog))
+isbin = "DFReader_binary" in mlog_type
+islog = "DFReader_text" in mlog_type # NOTE: "islog" does not mean a tlog
+istlog = "mavmmaplog" in mlog_type
 
 # 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*']