diff --git a/spec/util_spec.cr b/spec/util_spec.cr
index 94c326c..3ee4aac 100644
--- a/spec/util_spec.cr
+++ b/spec/util_spec.cr
@@ -35,6 +35,23 @@ describe "compare_numerically" do
   end
 end
 
+describe "is_supported_file" do
+  it "returns true when the filename has a supported extension" do
+    filename = "manga.cbz"
+    is_supported_file(filename).should eq true
+  end
+
+  it "returns true when the filename does not have a supported extension" do
+    filename = "info.json"
+    is_supported_file(filename).should eq false
+  end
+
+  it "is case insensitive" do
+    filename = "manga.ZiP"
+    is_supported_file(filename).should eq true
+  end
+end
+
 describe "chapter_sort" do
   it "sorts correctly" do
     ary = ["Vol.1 Ch.01", "Vol.1 Ch.02", "Vol.2 Ch. 2.5", "Ch. 3", "Ch.04"]
diff --git a/src/library/title.cr b/src/library/title.cr
index 001e9af..4c439e7 100644
--- a/src/library/title.cr
+++ b/src/library/title.cr
@@ -36,7 +36,7 @@ class Title
         @title_ids << title.id
         next
       end
-      if [".zip", ".cbz", ".rar", ".cbr"].includes? File.extname path
+      if is_supported_file path
         entry = Entry.new path, self
         @entries << entry if entry.pages > 0 || entry.err_msg
       end
diff --git a/src/util/signature.cr b/src/util/signature.cr
index 0db6b21..05eb62d 100644
--- a/src/util/signature.cr
+++ b/src/util/signature.cr
@@ -1,3 +1,5 @@
+require "./util"
+
 class File
   abstract struct Info
     def inode
@@ -11,7 +13,7 @@ class File
   #   ensures that moving (unless to another device) and renaming the file
   #   preserves the signature, while copying or editing the file changes it.
   def self.signature(filename) : UInt64
-    return 0u64 unless %w(.zip .rar .cbz .cbr).includes? File.extname filename
+    return 0u64 unless is_supported_file filename
     info = File.info filename
     signatures = [
       info.inode,
diff --git a/src/util/util.cr b/src/util/util.cr
index f174c39..873a226 100644
--- a/src/util/util.cr
+++ b/src/util/util.cr
@@ -2,6 +2,7 @@ IMGS_PER_PAGE            = 5
 ENTRIES_IN_HOME_SECTIONS = 8
 UPLOAD_URL_PREFIX        = "/uploads"
 STATIC_DIRS              = ["/css", "/js", "/img", "/favicon.ico"]
+SUPPORTED_FILE_EXTNAMES  = [".zip", ".cbz", ".rar", ".cbr"]
 
 def random_str
   UUID.random.to_s.gsub "-", ""
@@ -31,6 +32,10 @@ def register_mime_types
   end
 end
 
+def is_supported_file(path)
+  SUPPORTED_FILE_EXTNAMES.includes? File.extname(path).downcase
+end
+
 struct Int
   def or(other : Int)
     if self == 0