How to see which file Dropbox is currently uploading/downloading?

There's no easy way to tell from within Dropbox, but you can get an idea of it by looking at what files the dropbox process is accessing.

For any version of Windows, you can use SysInternals ProcMon. This gives a list of all current file, registry, and other actions. It can be filtered down to show just the data you want - in this case, what files Dropbox is accessing.

Open procmon.exe, and open the Filters dialog. Set the following filters:

Process Name   is           Dropbox.exe                     Include
Operation      is           ReadFile                        Include
Operation      is           WriteFile                       Include
Path           begins with  C:\users\YOURUSERNAME\AppData   Exclude
Path           is           C:                              Exclude

This will result in ProcMon only showing file reads and writes from Dropbox, excluding the config files that Dropbox accesses. Effectively, this will show you all files that Dropbox is uploading or downloading.

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A quicker but harder to interpret method is to use the Resource Monitor in Windows 7. To access the resource monitor, open the Task Manager (Ctrl+Shift+Esc), go to the Performance tab, and click on Resource Monitor.

Once that opens, go to the Disk tab, and in the 2nd panel, sort by the process name. Look for the activity under the Dropbox process to get an idea of what file it's processing.

You can use fs_usage on the Mac to view Dropbox's activity:

sudo fs_usage | grep ~/Dropbox

This uses grep to search for any activity with my User's Dropbox folder. You could also try the following to view all activity:

sudo fs_usage -f filesys

Use the following to only show activity from the Dropbox process (if Dropbox is the name of the process):

sudo fs_usage Dropbox

Hope that helps :)

For Linux, you can probably get a hint by looking at what files the Dropbox process is accessing.

ls -l /proc/$(pidof dropbox)/fd | egrep -v 'pipe:|socket:|/dev'

Just because Dropbox is accessing a file, doesn't necessarily mean it's being uploaded. With a bit of human oversight however, you should be able to make a good guess about which file (if any) is really being uploaded.