import: Don't assume a last_modified key is present.
import: Don't assume a last_modified key is present.

This fixes an exception when importing uploaded file data from
Slack/HipChat.

zulip/zulip,dhcrzf/zulip,timabbott/zulip,rht/zulip,timabbott/zulip,andersk/zulip,shubhamdhama/zulip,kou/zulip,brainwane/zulip,punchagan/zulip,eeshangarg/zulip,andersk/zulip,jackrzhang/zulip,tommyip/zulip,zulip/zulip,kou/zulip,rht/zulip,eeshangarg/zulip,synicalsyntax/zulip,jackrzhang/zulip,rishig/zulip,kou/zulip,eeshangarg/zulip,rishig/zulip,synicalsyntax/zulip,andersk/zulip,timabbott/zulip,jackrzhang/zulip,brainwane/zulip,andersk/zulip,showell/zulip,hackerkid/zulip,brainwane/zulip,showell/zulip,showell/zulip,rishig/zulip,showell/zulip,punchagan/zulip,synicalsyntax/zulip,punchagan/zulip,rishig/zulip,timabbott/zulip,brainwane/zulip,dhcrzf/zulip,timabbott/zulip,tommyip/zulip,tommyip/zulip,zulip/zulip,hackerkid/zulip,hackerkid/zulip,tommyip/zulip,dhcrzf/zulip,showell/zulip,jackrzhang/zulip,rht/zulip,punchagan/zulip,eeshangarg/zulip,dhcrzf/zulip,zulip/zulip,zulip/zulip,timabbott/zulip,punchagan/zulip,tommyip/zulip,eeshangarg/zulip,brainwane/zulip,hackerkid/zulip,shubhamdhama/zulip,rishig/zulip,rht/zulip,jackrzhang/zulip,punchagan/zulip,shubhamdhama/zulip,rishig/zulip,hackerkid/zulip,tommyip/zulip,eeshangarg/zulip,andersk/zulip,shubhamdhama/zulip,hackerkid/zulip,hackerkid/zulip,rht/zulip,kou/zulip,jackrzhang/zulip,zulip/zulip,brainwane/zulip,shubhamdhama/zulip,timabbott/zulip,eeshangarg/zulip,shubhamdhama/zulip,synicalsyntax/zulip,shubhamdhama/zulip,kou/zulip,brainwane/zulip,andersk/zulip,dhcrzf/zulip,synicalsyntax/zulip,andersk/zulip,rishig/zulip,zulip/zulip,punchagan/zulip,showell/zulip,synicalsyntax/zulip,kou/zulip,synicalsyntax/zulip,dhcrzf/zulip,rht/zulip,rht/zulip,jackrzhang/zulip,showell/zulip,kou/zulip,dhcrzf/zulip,tommyip/zulip