Index: app/models/attachment.rb
===================================================================
--- app/models/attachment.rb	(revision 17462)
+++ app/models/attachment.rb	(working copy)
@@ -235,7 +235,12 @@
   end
 
   def is_text?
-    Redmine::MimeType.is_type?('text', filename)
+    if Redmine::MimeType.is_type?('text', filename)
+      true
+    else
+      data = File.read(diskfile, 4096) rescue false
+      ! Redmine::Utils.binary?(data)
+    end
   end
 
   def is_image?
Index: lib/redmine/scm/adapters/abstract_adapter.rb
===================================================================
--- lib/redmine/scm/adapters/abstract_adapter.rb	(revision 17462)
+++ lib/redmine/scm/adapters/abstract_adapter.rb	(working copy)
@@ -425,9 +425,7 @@
 
       module ScmData
         def self.binary?(data)
-          unless data.empty?
-            data.count( "^ -~", "^\r\n" ).fdiv(data.size) > 0.3 || data.index( "\x00" )
-          end
+          Redmine::Utils.binary?(data)
         end
       end
     end
Index: lib/redmine/utils.rb
===================================================================
--- lib/redmine/utils.rb	(revision 17462)
+++ lib/redmine/utils.rb	(working copy)
@@ -61,6 +61,12 @@
           end
         end
       end
+
+      def binary?(data)
+        unless data.empty?
+          data.count( "^ -~", "^\r\n" ).fdiv(data.size) > 0.3 || data.index( "\x00" )
+        end
+      end
     end
 
     module Shell
Index: test/fixtures/files/hello.go
===================================================================
--- test/fixtures/files/hello.go	(nonexistent)
+++ test/fixtures/files/hello.go	(working copy)
@@ -0,0 +1,7 @@
+package main
+
+import "fmt"
+
+func main() {
+  fmt.Println("Hello, world!")
+}
Index: test/unit/attachment_test.rb
===================================================================
--- test/unit/attachment_test.rb	(revision 17462)
+++ test/unit/attachment_test.rb	(working copy)
@@ -444,4 +444,24 @@
     puts '(ImageMagick convert not available)'
   end
 
+  def test_is_text
+    files = [
+      {:name => 'testfile.txt', :type => 'text/plain'},
+      {:name => 'hello.go', :type => 'text/plain'},
+      {:name => '2006/07/060719210727_archive.zip', :type => 'application/octet-stream'}
+    ]
+
+    a_text, a_go, a_bin = files.map do |f|
+      a = Attachment.new(:container => Issue.find(1),
+        :file => uploaded_test_file(f[:name], f[:type]),
+        :author => User.find(1))
+      a.save!
+      a
+    end
+
+    assert a_text.is_text?
+    assert a_go.is_text?
+    assert_not a_bin.is_text?
+  end
+
 end