Handle File.extname edge case

src/file.cr

@@ -282,35 +282,37 @@ class File < IO::FileDescriptor
   def self.extname(filename) : String
     filename.check_no_null_byte
 
-    reader = Char::Reader.new(at_end: filename)
+    bytes = filename.to_slice
+    current = bytes.size - 1
 
     # if the pattern is foo. it has no extension
-    return "" if reader.current_char == '.'
+    return "" if bytes[current] == '.'.ord
 
     # position the reader at the last . or SEPARATOR
-    while (current_char = reader.current_char) &&
-          (current_char != SEPARATOR && current_char != '.') &&
-          reader.has_previous?
-      reader.previous_char
+    # that is not the first char
+    while bytes[current] != SEPARATOR.ord &&
+          bytes[current] != '.'.ord &&
+          current > 0
+      current -= 1
     end
 
     # if we are the beginning of the string there is no extension
     # /foo or .foo have no extension
-    return "" unless reader.has_previous?
+    return "" unless current > 0
 
     # otherwise we are not at the beginning, and there is a previous char.
     # if current is '/', then the pattern is prefix/foo and has no extension
-    return "" if current_char == SEPARATOR
+    return "" if bytes[current] == SEPARATOR.ord
 
     # otherwise the current_char is '.'
     # if previous is '/', then the pattern is prefix/.foo  and has no extension
-    return "" if reader.previous_char == SEPARATOR
+    return "" if bytes[current - 1] == SEPARATOR.ord
 
     # So the current char is '.',
     # we are not at the beginning,
     # the previous char is not a '/',
     # and we have an extension
-    return filename.byte_slice(reader.pos + 1)
+    String.new(bytes[current, bytes.size - current])
   end
 
   # Converts *path* to an absolute path. Relative paths are