I recently had a Gmail image download lock up on my Android, with the download continuing for nearly an hour, even after the image was safely stored in the download folder. When turning off mobile data and Wi-Fi, doing several restarts, and removing the battery proved ineffectual (although after each restart, it was a couple minutes before the download resumed), I searched the web for an answer. Efforts to find an online solution proved equally fruitless, as they generally applied specifically to app downloads. In addition to the inconvenience, I assumed that–as long as the download persisted–it would also continue to drain the battery.
So, the following steps (in Android 4.0) were taken and the download finally stopped.
Below, you can see the download running.
In the Device section of Settings, open Apps.
From there, select All apps (upper right), scroll down, and tap Download Manager. You can see the download running in the background (upper left).
As the apps cache had already been cleared with the History Eraser,  I tapped the Clear Data button.
The warning below caused some consternation, but something had to be done, so I tapped OK.
For people running an earlier Android version (Gingerbread is displayed below), the steps are similar.
Tap the menu button and select Settings. In Settings, choose Applications.
In Application settings, tap Manage applications.
Scroll down, open the Download Manager, and tap the Clear data button.
After clearing the data, the download stopped, new downloads are completing error free, birds are singing, and all seems right with the world.