Catch AttributeError when deleting temp file on image upload
Catch AttributeError when deleting temp file on image upload

When the image upload finishes, we try to delete the temporary
file that was used for the upload. However, depending on how
Horizon is deployed, instead of a file it can be a BytesIO
instance, which leads to an AttributeError. This happens in
a separate thread, so Horizon itself doesn't crash, but it's
better to catch that exception.

Change-Id: Iaa66d9c87060c1e001f3f6e31f630d6092df55d3
Closes-Bug: #1866632

openstack/horizon,ChameleonCloud/horizon,ChameleonCloud/horizon,openstack/horizon,openstack/horizon,openstack/horizon,ChameleonCloud/horizon,ChameleonCloud/horizon