diff --git a/scrapy/commands/crawl.py b/scrapy/commands/crawl.py
index f0cd06a55..1cd40469b 100644
--- a/scrapy/commands/crawl.py
+++ b/scrapy/commands/crawl.py
@@ -1,7 +1,9 @@
+import os
 from scrapy.command import ScrapyCommand
 from scrapy.utils.conf import arglist_to_dict
 from scrapy.exceptions import UsageError
 
+
 class Command(ScrapyCommand):
 
     requires_project = True
@@ -14,12 +16,12 @@ class Command(ScrapyCommand):
 
     def add_options(self, parser):
         ScrapyCommand.add_options(self, parser)
-        parser.add_option("-a", dest="spargs", action="append", default=[], metavar="NAME=VALUE", \
-            help="set spider argument (may be repeated)")
-        parser.add_option("-o", "--output", metavar="FILE", \
-            help="dump scraped items into FILE (use - for stdout)")
-        parser.add_option("-t", "--output-format", metavar="FORMAT", default="jsonlines", \
-            help="format to use for dumping items with -o (default: %default)")
+        parser.add_option("-a", dest="spargs", action="append", default=[], metavar="NAME=VALUE",
+                          help="set spider argument (may be repeated)")
+        parser.add_option("-o", "--output", metavar="FILE",
+                          help="dump scraped items into FILE (use - for stdout)")
+        parser.add_option("-t", "--output-format", metavar="FORMAT",
+                          help="format to use for dumping items with -o")
 
     def process_options(self, args, opts):
         ScrapyCommand.process_options(self, args, opts)
@@ -33,8 +35,13 @@ class Command(ScrapyCommand):
             else:
                 self.settings.overrides['FEED_URI'] = opts.output
             valid_output_formats = self.settings['FEED_EXPORTERS'].keys() + self.settings['FEED_EXPORTERS_BASE'].keys()
+            if not opts.output_format:
+                opts.output_format = os.path.splitext(opts.output)[1].replace(".", "")
             if opts.output_format not in valid_output_formats:
-                raise UsageError('Invalid/unrecognized output format: %s, Expected %s' % (opts.output_format, valid_output_formats))
+                raise UsageError("Unrecognized output format '%s', set one"
+                                 " using the '-t' switch or as a file extension"
+                                 " from the supported list %s" % (opts.output_format,
+                                                                  tuple(valid_output_formats)))
             self.settings.overrides['FEED_FORMAT'] = opts.output_format
 
     def run(self, args, opts):
diff --git a/scrapy/commands/runspider.py b/scrapy/commands/runspider.py
index 14d67e24b..2d537dee7 100644
--- a/scrapy/commands/runspider.py
+++ b/scrapy/commands/runspider.py
@@ -7,6 +7,7 @@ from scrapy.command import ScrapyCommand
 from scrapy.exceptions import UsageError
 from scrapy.utils.conf import arglist_to_dict
 
+
 def _import_file(filepath):
     abspath = os.path.abspath(filepath)
     dirname, file = os.path.split(abspath)
@@ -22,6 +23,7 @@ def _import_file(filepath):
             sys.path.pop(0)
     return module
 
+
 class Command(ScrapyCommand):
 
     requires_project = False
@@ -37,12 +39,12 @@ class Command(ScrapyCommand):
 
     def add_options(self, parser):
         ScrapyCommand.add_options(self, parser)
-        parser.add_option("-a", dest="spargs", action="append", default=[], metavar="NAME=VALUE", \
-            help="set spider argument (may be repeated)")
-        parser.add_option("-o", "--output", metavar="FILE", \
-            help="dump scraped items into FILE (use - for stdout)")
-        parser.add_option("-t", "--output-format", metavar="FORMAT", default="jsonlines", \
-            help="format to use for dumping items with -o (default: %default)")
+        parser.add_option("-a", dest="spargs", action="append", default=[], metavar="NAME=VALUE",
+                          help="set spider argument (may be repeated)")
+        parser.add_option("-o", "--output", metavar="FILE",
+                          help="dump scraped items into FILE (use - for stdout)")
+        parser.add_option("-t", "--output-format", metavar="FORMAT",
+                          help="format to use for dumping items with -o")
 
     def process_options(self, args, opts):
         ScrapyCommand.process_options(self, args, opts)
@@ -56,8 +58,13 @@ class Command(ScrapyCommand):
             else:
                 self.settings.overrides['FEED_URI'] = opts.output
             valid_output_formats = self.settings['FEED_EXPORTERS'].keys() + self.settings['FEED_EXPORTERS_BASE'].keys()
+            if not opts.output_format:
+                opts.output_format = os.path.splitext(opts.output)[1].replace(".", "")
             if opts.output_format not in valid_output_formats:
-                raise UsageError('Invalid/unrecognized output format: %s, Expected %s' % (opts.output_format,valid_output_formats))
+                raise UsageError("Unrecognized output format '%s', set one"
+                                 " using the '-t' switch or as a file extension"
+                                 " from the supported list %s" % (opts.output_format,
+                                                                  tuple(valid_output_formats)))
             self.settings.overrides['FEED_FORMAT'] = opts.output_format
 
     def run(self, args, opts):