diff --git a/lib/derivative_rodeo/generators/thumbnail_generator.rb b/lib/derivative_rodeo/generators/thumbnail_generator.rb
index 70b3bd9..af533e2 100644
--- a/lib/derivative_rodeo/generators/thumbnail_generator.rb
+++ b/lib/derivative_rodeo/generators/thumbnail_generator.rb
@@ -50,13 +50,8 @@ def build_step(output_location:, input_tmp_file_path:, **)
       #
       # @see .dimensions_by_type
       # @see .dimensions_fallback
-      #
-      # @note TODO: This is a very quick and dirty and assumptive type detector.  For the 2023-05-31
-      #       use case it is likely adequate (e.g. if it ends in .pdf we'll have a configured
-      #       match).  In other words, we'd love someone else to be sniffing out mime-types rather
-      #       than doing it here.
       def self.dimensions_for(filename:)
-        type = filename.split(".")&.last&.to_sym
+        type = DerivativeRodeo::Services::MimeTypeService.hyrax_type(filename: filename)
         dimensions_by_type.fetch(type, dimensions_fallback)
       end
 
diff --git a/lib/derivative_rodeo/services/mime_type_service.rb b/lib/derivative_rodeo/services/mime_type_service.rb
new file mode 100644
index 0000000..149c3ff
--- /dev/null
+++ b/lib/derivative_rodeo/services/mime_type_service.rb
@@ -0,0 +1,52 @@
+# frozen_string_literal: true
+require 'marcel'
+
+module DerivativeRodeo
+  module Services
+    ##
+    # This module provides an interface for determining a mime-type.
+    module MimeTypeService
+      ##
+      # Hyrax has it's own compression of mime_types into conceptual types (as defined in
+      # Hyrax::FileSetDerivativesService).  This provides a somewhat conceptual overlap with that,
+      # while also being more generalized.
+      #
+      # @param filename [String]
+      # @return [Symbol]
+      def self.hyrax_type(filename:)
+        mime = mime_type(filename: filename)
+        media_type, sub_type = mime.split("/")
+        case media_type
+        when "image", "audio", "text", "video"
+          media_type.to_sym
+        when "application" # The wild woolly weird world of all the things.
+          # TODO: Do we need to worry about office documents?
+          sub_type.to_sym
+        else
+          sub_type.to_sym
+        end
+      end
+
+      ##
+      # Given a local :filename (e.g. downloaded and available on the server this is running),
+      # return the mime_type of the file.
+      #
+      # @param filename [String]
+      # @return [String] (e.g. "application/pdf", "text/plain")
+      def self.mime_type(filename:)
+        ##
+        # TODO: Does this attempt to read the whole file?  That may create memory constraints.  By
+        # using Pathname (instead of File.read), we're letting Marcel do it's best mime magic.
+        pathname = Pathname.new(filename)
+        extension = filename.split(".")&.last&.downcase
+        if extension
+          # By including a possible extension, we can help nudge Marcel into making a more
+          # Without extension, we will get a lot of "application/octet-stream" results.
+          ::Marcel::MimeType.for(pathname, extension: extension)
+        else
+          ::Marcel::MimeType.for(pathname)
+        end
+      end
+    end
+  end
+end
diff --git a/spec/derivative_rodeo/generators/thumbnail_generator_spec.rb b/spec/derivative_rodeo/generators/thumbnail_generator_spec.rb
index 586b04e..9d8d5b4 100644
--- a/spec/derivative_rodeo/generators/thumbnail_generator_spec.rb
+++ b/spec/derivative_rodeo/generators/thumbnail_generator_spec.rb
@@ -22,18 +22,13 @@
   describe '.dimensions_for' do
     subject { described_class.dimensions_for(filename: filename) }
 
-    context "given a file ending in '.pdf'" do
-      let(:filename) { "really-cool.pdf" }
+    context "given a PDF" do
+      let(:filename) { Fixtures.path_for("minimal-2-page.pdf") }
       it { is_expected.to eq described_class.dimensions_by_type.fetch(:pdf) }
     end
 
-    context "given a file without an extension" do
-      let(:filename) { "aint-no-extension-here" }
-      it { is_expected.to eq described_class.dimensions_fallback }
-    end
-
-    context "given a file ending in '.tiff'" do
-      let(:filename) { "muppet-man.tiff" }
+    context "given a TIFF" do
+      let(:filename) { Fixtures.path_for("4.1.07.tiff") }
       it { is_expected.to eq described_class.dimensions_fallback }
     end
   end
diff --git a/spec/derivative_rodeo/services/mime_type_service_spec.rb b/spec/derivative_rodeo/services/mime_type_service_spec.rb
new file mode 100644
index 0000000..aa093f3
--- /dev/null
+++ b/spec/derivative_rodeo/services/mime_type_service_spec.rb
@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe DerivativeRodeo::Services::MimeTypeService do
+  describe '.mime_type' do
+    subject { described_class.mime_type(filename: filename) }
+    {
+      __FILE__ => "text/x-ruby",
+      Fixtures.path_for('4.1.07.tiff') => "image/tiff",
+      Fixtures.path_for('tiff-no-ext') => "image/tiff",
+      Fixtures.path_for('minimal-1-page.pdf') => "application/pdf",
+      Fixtures.path_for('ndnp-sample1-txt.txt') => "text/plain"
+    }.each do |given_filename, expected_mime_type|
+      context "for #{File.basename(given_filename)}" do
+        let(:filename) { given_filename }
+        it { is_expected.to eq(expected_mime_type) }
+      end
+    end
+  end
+
+  describe '.hyrax_type' do
+    subject { described_class.hyrax_type(filename: filename) }
+    {
+      Fixtures.path_for('4.1.07.tiff') => :image,
+      Fixtures.path_for('minimal-1-page.pdf') => :pdf,
+      Fixtures.path_for('ndnp-sample1-txt.txt') => :text
+    }.each do |given_filename, expected_hyrax_type|
+      context "for #{File.basename(given_filename)}" do
+        let(:filename) { given_filename }
+        it { is_expected.to eq(expected_hyrax_type) }
+      end
+    end
+  end
+end
diff --git a/spec/fixtures/files/tiff-no-ext b/spec/fixtures/files/tiff-no-ext
new file mode 100644
index 0000000..97ca385
Binary files /dev/null and b/spec/fixtures/files/tiff-no-ext differ