An upload progress mod has been added in the forum so that a progress bar appears in the attachment section giving an idea of how much has been uploaded and an estimate on how long is left. This requires JavaScript to be enabled, but otherwise requires no additional browser settings or plugins.
Here's an example of how it looks:
For small files you may not see it as it updates every 100kB.
If it proves to work OK I'll look at possibly adding it to the PM system, gallery, and anywhere else uploads to the site occur.