FIX: Incorrect/missing extension in short_url fails to map to upload.

`Hash#invert` causes us to lose keys if the hash contains similar
values.

lib/pretty_text/helpers.rb

@@ -63,13 +63,18 @@ module PrettyText
       end
 
       if map.length > 0
-        reverse_map = map.invert
+        reverse_map = {}
+
+        map.each do |key, value|
+          reverse_map[value] ||= []
+          reverse_map[value] << key
+        end
 
         Upload.where(sha1: map.values).pluck(:sha1, :url).each do |row|
           sha1, url = row
 
-          if short_url = reverse_map[sha1]
-            result[short_url] = url
+          if short_urls = reverse_map[sha1]
+            short_urls.each { |short_url| result[short_url] = url }
           end
         end
       end

spec/components/pretty_text_spec.rb

@@ -1228,6 +1228,8 @@ HTML
 
       - test
           - ![upload](#{upload.short_url})
+
+      ![upload](#{upload.short_url.gsub!(".png", "")})
       RAW
 
       cooked = <<~HTML
@@ -1243,6 +1245,7 @@ HTML
         </ul>
         </li>
         </ul>
+        <p><img src="#{upload.url}" alt="upload"></p>
       HTML
 
       expect(PrettyText.cook(raw)).to eq(cooked.strip)