If anyone's interested: I've filed a patch which allows the inline module to also access files uploaded with attachment module.
Used in combination you're now able to inject documents, files and images to your contents (nodes) with the benefits of the attachment module (title, description, thread safety, disk space control).

Additionally it is supporting mime file type icons now (as long as the theme is providing these in a subdirectory mime/).

Patch is for review but already working on one of our sites.

Note: Functionality of Drupal core upload module is not affected by this patch.

Just install both and start injecting your files.