import: Guess upload content-types when unavailable from export.
import: Guess upload content-types when unavailable from export.

This is mostly for exports from other software like Slack, that might
not provide a content-type.

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