GMail attachment download does not complete
Though not the identical issue, the solution provided with the question Download stuck in Gmail solves it as well:

Clear the cache of the Download manager. To do so, go to Settings&rarr;Apps&rarr;Manage Apps, switch to the "All" Tab, scroll to the "Download Manager". Tap that entry, and hit the button labeled "Clear Cache". Optionally, also tap "Clear Data", if cache-cleaning alone does not solve the issue.