diff --git a/app/models/zip_file/reader.rb b/app/models/zip_file/reader.rb
index 762e248de..1b9cd3dcb 100644
--- a/app/models/zip_file/reader.rb
+++ b/app/models/zip_file/reader.rb
@@ -10,7 +10,7 @@ class ZipFile::Reader
     raise ArgumentError, "Cannot read directory entry: #{file_path}" if entry.filename.end_with?("/")
 
     if block_given?
-      yield ZipFile::Reader::ExtractorIO.new(entry, @io)
+      yield ZipFile::Reader::IO.new(entry, @io)
     else
       entry.extractor_from(@io).extract
     end
diff --git a/app/models/zip_file/reader/extractor_io.rb b/app/models/zip_file/reader/io.rb
similarity index 93%
rename from app/models/zip_file/reader/extractor_io.rb
rename to app/models/zip_file/reader/io.rb
index 13640f445..cf28212aa 100644
--- a/app/models/zip_file/reader/extractor_io.rb
+++ b/app/models/zip_file/reader/io.rb
@@ -1,4 +1,4 @@
-class ZipFile::Reader::ExtractorIO
+class ZipFile::Reader::IO
   def initialize(entry, io)
     @entry = entry
     @io = io
diff --git a/app/models/zip_file/writer.rb b/app/models/zip_file/writer.rb
index 144416c79..8e9665563 100644
--- a/app/models/zip_file/writer.rb
+++ b/app/models/zip_file/writer.rb
@@ -10,7 +10,7 @@ class ZipFile::Writer
   end
 
   def io
-    @counting_io ||= CountingIO.new(self)
+    @io ||= ZipFile::Writer::IO.new(self)
   end
 
   def stream_to(io)
@@ -55,19 +55,4 @@ class ZipFile::Writer
     def streamer
       @streamer ||= ZipKit::Streamer.new(@output_io)
     end
-
-  class CountingIO
-    def initialize(writer)
-      @writer = writer
-    end
-
-    def write(data)
-      @writer.write(data)
-    end
-
-    def <<(data)
-      write(data)
-      self
-    end
-  end
 end
diff --git a/app/models/zip_file/writer/io.rb b/app/models/zip_file/writer/io.rb
new file mode 100644
index 000000000..451dd0d07
--- /dev/null
+++ b/app/models/zip_file/writer/io.rb
@@ -0,0 +1,18 @@
+class ZipFile::Writer::IO
+  def initialize(writer)
+    @writer = writer
+  end
+
+  def write(data)
+    @writer.write(data)
+  end
+
+  def <<(data)
+    write(data)
+    self
+  end
+
+  def size
+    @writer.byte_size
+  end
+end
diff --git a/test/models/zip_file_test.rb b/test/models/zip_file_test.rb
new file mode 100644
index 000000000..257b9adf4
--- /dev/null
+++ b/test/models/zip_file_test.rb
@@ -0,0 +1,141 @@
+require "test_helper"
+
+class ZipFileTest < ActiveSupport::TestCase
+  test "writer io exposes size for S3 multipart upload detection" do
+    writer = ZipFile::Writer.new
+
+    assert_respond_to writer.io, :size
+    assert_equal writer.byte_size, writer.io.size
+  end
+
+  test "writer adds files with content" do
+    tempfile = Tempfile.new([ "test", ".zip" ])
+    tempfile.binmode
+
+    writer = ZipFile::Writer.new(tempfile)
+    writer.add_file("hello.txt", "Hello, World!")
+    writer.close
+
+    assert writer.exists?("hello.txt")
+    assert_not writer.exists?("missing.txt")
+  end
+
+  test "writer adds files with block" do
+    tempfile = Tempfile.new([ "test", ".zip" ])
+    tempfile.binmode
+
+    writer = ZipFile::Writer.new(tempfile)
+    writer.add_file("hello.txt") { |sink| sink.write("Hello, World!") }
+    writer.close
+
+    assert writer.exists?("hello.txt")
+  end
+
+  test "writer globs entries" do
+    tempfile = Tempfile.new([ "test", ".zip" ])
+    tempfile.binmode
+
+    writer = ZipFile::Writer.new(tempfile)
+    writer.add_file("docs/readme.txt", "Readme")
+    writer.add_file("docs/guide.txt", "Guide")
+    writer.add_file("images/logo.png", "PNG data")
+    writer.close
+
+    assert_equal [ "docs/guide.txt", "docs/readme.txt" ], writer.glob("docs/*.txt")
+    assert_equal [ "images/logo.png" ], writer.glob("**/*.png")
+  end
+
+  test "reader reads file content" do
+    tempfile = create_test_zip("hello.txt" => "Hello, World!")
+
+    reader = ZipFile::Reader.new(tempfile)
+    content = reader.read("hello.txt")
+
+    assert_equal "Hello, World!", content
+  end
+
+  test "reader reads file with block" do
+    tempfile = create_test_zip("hello.txt" => "Hello, World!")
+
+    reader = ZipFile::Reader.new(tempfile)
+    content = nil
+    reader.read("hello.txt") { |io| content = io.read }
+
+    assert_equal "Hello, World!", content
+  end
+
+  test "reader raises for missing file" do
+    tempfile = create_test_zip("hello.txt" => "Hello")
+
+    reader = ZipFile::Reader.new(tempfile)
+
+    assert_raises(ArgumentError) { reader.read("missing.txt") }
+  end
+
+  test "reader checks file existence" do
+    tempfile = create_test_zip("hello.txt" => "Hello")
+
+    reader = ZipFile::Reader.new(tempfile)
+
+    assert reader.exists?("hello.txt")
+    assert_not reader.exists?("missing.txt")
+  end
+
+  test "reader globs entries" do
+    tempfile = create_test_zip(
+      "docs/readme.txt" => "Readme",
+      "docs/guide.txt" => "Guide",
+      "images/logo.png" => "PNG"
+    )
+
+    reader = ZipFile::Reader.new(tempfile)
+
+    assert_equal [ "docs/guide.txt", "docs/readme.txt" ], reader.glob("docs/*.txt")
+  end
+
+  test "reader io provides size" do
+    tempfile = create_test_zip("hello.txt" => "Hello, World!")
+
+    reader = ZipFile::Reader.new(tempfile)
+    reader.read("hello.txt") do |io|
+      assert_equal 13, io.size
+    end
+  end
+
+  test "reader io supports rewind" do
+    tempfile = create_test_zip("hello.txt" => "Hello, World!")
+
+    reader = ZipFile::Reader.new(tempfile)
+    reader.read("hello.txt") do |io|
+      first_read = io.read
+      io.rewind
+      second_read = io.read
+
+      assert_equal first_read, second_read
+    end
+  end
+
+  test "reader io tracks eof" do
+    tempfile = create_test_zip("hello.txt" => "Hello")
+
+    reader = ZipFile::Reader.new(tempfile)
+    reader.read("hello.txt") do |io|
+      assert_not io.eof?
+      io.read
+      assert io.eof?
+    end
+  end
+
+  private
+    def create_test_zip(files)
+      tempfile = Tempfile.new([ "test", ".zip" ])
+      tempfile.binmode
+
+      writer = ZipFile::Writer.new(tempfile)
+      files.each { |path, content| writer.add_file(path, content) }
+      writer.close
+
+      tempfile.rewind
+      tempfile
+    end
+end