Return nil rather than raising an error

lib/gitlab/utils/mime_type.rb

@@ -5,17 +5,15 @@
 module Gitlab
   module Utils
     class MimeType
-      Error = Class.new(StandardError)
-
       class << self
         def from_io(io)
-          raise Error, "input isn't an IO object" unless io.is_a?(IO) || io.is_a?(StringIO)
+          return unless io.is_a?(IO) || io.is_a?(StringIO)
 
           MimeMagic.by_magic(io).try(:type)
         end
 
         def from_string(string)
-          raise Error, "input isn't a string" unless string.is_a?(String)
+          return unless string.is_a?(String)
 
           MimeMagic.by_magic(string)
         end

spec/lib/gitlab/utils/mime_type_spec.rb

@@ -10,8 +10,8 @@ RSpec.describe Gitlab::Utils::MimeType do
     context "input isn't an IO" do
       let(:io) { "test" }
 
-      it "raises an error" do
-        expect { subject }.to raise_error(Gitlab::Utils::MimeType::Error)
+      it "returns nil" do
+        expect(subject).to be_nil
       end
     end
 
@@ -42,8 +42,8 @@ RSpec.describe Gitlab::Utils::MimeType do
     context "input isn't a string" do
       let(:str) { nil }
 
-      it "raises an error" do
-        expect { subject }.to raise_error(Gitlab::Utils::MimeType::Error)
+      it "returns nil" do
+        expect(subject).to be_nil
       end
     end