Redmine

2nd try:

In our redmine installation, we attach HTML documentation files under project Documentation section (such as asciidoc-generated html code). The problem is, redmine currently forces Firefox browser to open the download dialog instead to open HTML inline in the browser.

As far I traced this down, this is due to setting the Content-Disposition flag to "attachment" by redmine. I could fix this by adding test for is_text mimetype in in attachments_controller.rb:

@ ---snip---
def download
if @attachment.container.is_a?(Version) || @attachment.container.is_a?(Project)
@attachment.increment_download
end

1. images are sent inline
   send_file @attachment.diskfile, :filename => filename_for_content_disposition(@attachment.filename),
   :type => @attachment.content_type,
2. :disposition => (@attachment.image? ? 'inline' : 'attachment')
   :disposition => ((@attachment.image? || @attachment.is_text?) ? 'inline' : 'attachment')
   end

---snip---
@ 
However, I think this should be handled more in general since there might be pretty much other mimetypes where this is a problem as well.