Fix #3910: Corrupted images during upload.

Use a fresh tempfile for each download attempt instead of reusing the same
file (and having to rewind/truncate it after each failed attempt).

test/unit/downloads/file_test.rb

@@ -6,7 +6,6 @@ module Downloads
       setup do
         @source = "http://www.google.com/intl/en_ALL/images/logo.gif"
         @download = Downloads::File.new(@source)
-        @tempfile = Tempfile.new("danbooru-test")
       end
 
       context "that fails" do
@@ -14,11 +13,22 @@ module Downloads
           HTTParty.expects(:get).times(3).raises(Errno::ETIMEDOUT)
           assert_raises(Errno::ETIMEDOUT) { @download.download! }
         end
+
+        should "return an uncorrupted file on the second try" do
+          bomb = stub("bomb")
+          bomb.expects(:size).raises(IOError)
+          resp = stub("resp", success?: true)
+
+          HTTParty.expects(:get).twice.multiple_yields("a", bomb, "b", "c").then.multiple_yields("a", "b", "c").returns(resp)
+          tempfile, _ = @download.download!
+
+          assert_equal("abc", tempfile.read)
+        end
       end
 
       should "throw an exception when the file is larger than the maximum" do
         assert_raise(Downloads::File::Error) do
-          @download.http_get_streaming(@source, @tempfile, {}, max_size: 1)
+          @download.download!(max_size: 1)
         end
       end