DEV: Extend plugin API for uploads (#8440)

* DEV: Add API to alter uploads Markdown

* DEV: Extract data attributes from image / download Markdown

For example '[test|attachment|hello=world]' will generate an 'a' element
with a data attribute: 'data-hello=world'.

This commit also makes MarkdownIt to transform '|attachment' into
'class="attachment"'. This transformation used to be a part of the
process which resolves short URLs (i.e. upload://).

* DEV: Export imageNameFromFileName

test/javascripts/acceptance/composer-attachment-test.js.es6

@@ -34,6 +34,6 @@ QUnit.test("attachments are cooked properly", async assert => {
     find(".d-editor-preview:visible")
       .html()
       .trim(),
-    '<p><a href="/uploads/short-url/asdsad.png" class="attachment">test</a></p>'
+    '<p><a class="attachment" href="/uploads/short-url/asdsad.png">test</a></p>'
   );
 });