In 2.9.2, when clicking on a link to an attachment in an HTML export, the appropiate reader/application is launched based on the filetype. In 2.10 this is not working because the attachments do not have a filetype due to the current naming convention of all numbers with no extension. This is causing browsers to complain and is annoying for end users. Instead of the appropiate application/reader opening the attachment, firefox opens a prompt:
"You have chosen to open 6127634 which is a: application/octet-stream .... What should Firefox do with this file?" A similar prompt is displayed in IE.

Attachment link's html source from a 2.9.2 HTML export:

<div class="greybox" align="left">
<img src="images/icons/bullet_blue.gif" height="8" width="8" alt=""/>
<a href="download/attachments/5963819/Word2007userguide.docx">Word2007userguide.docx</a> (application/vnd.openxmlformats-officedocument.wordprocessingml.document)

Attachment link's html source from a 2.10 HTML export:

<div class="greybox" align="left">
<img src="images/icons/bullet_blue.gif" height="8" width="8" alt=""/>
<a href="attachments/5963819/6127634">Word2007userguide.docx</a> (application/vnd.openxmlformats-officedocument.wordprocessingml.document)

It works perfectly fine for jpegs, gifs, but seems to fail on anything else that doesn't open up in web browser normally.