add and apply util method is_interesting_file

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).downcase
+      if is_interesting_file path
         entry = Entry.new path, self
         @entries << entry if entry.pages > 0 || entry.err_msg
       end

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_interesting_file filename
     info = File.info filename
     signatures = [
       info.inode,

src/util/util.cr

@@ -1,7 +1,8 @@
-IMGS_PER_PAGE            = 5
-ENTRIES_IN_HOME_SECTIONS = 8
-UPLOAD_URL_PREFIX        = "/uploads"
-STATIC_DIRS              = ["/css", "/js", "/img", "/favicon.ico"]
+IMGS_PER_PAGE             = 5
+ENTRIES_IN_HOME_SECTIONS  = 8
+UPLOAD_URL_PREFIX         = "/uploads"
+STATIC_DIRS               = ["/css", "/js", "/img", "/favicon.ico"]
+INTERESTING_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_interesting_file(path)
+  INTERESTING_FILE_EXTNAMES.includes? (File.extname path).downcase
+end
+
 struct Int
   def or(other : Int)
     if self == 0