Sometimes, the delay in initial processing time for a very large file can cause issues with an upload. This problem becomes more severe if the file is large and it is being copied to many "child" messages from a single "parent" message. In these cases, you will want to turn on or enable Async uploads.
You can see if Async is turned on in the Organization sub-tab of the ADMIN tab.
How to turn ON (Enable) Async Upload
1. Click the Admin tab.
2. Click the Modify button under Organization.
3. Click the Misc. tab.
4. Make sure the slider next to Asynchronous upload is slid to ON as shown to enable Async.
Slide the slider to "OFF" to disable Async.
5. Click SAVE.