Add support for deleting files from slack to discord. Fixes #1705 (#1709)

We create a new event EventFileDelete which will be used to delete
specific uploaded files using the Extra["file"] in the config.Message.

We also add a new NativeID key to the FileInfo struct which will contain
the native file ID of the sending bridge.

When a new file is added to the config.Message.Extra["file"] map, now
the bridge native file ID should be added here.

When the receiving bridge receives such a message, it should keep an
internal mapping of NativeID <> bridge fileid/message id. In the case of
discord we map it to the resulted discord message ID after uploading it.

Now when a bridge deletes a file, it should send a EventFileDelete and
setting the ID to the native file ID of the bridge.

When the receiving bridge will get this event it'll look into the
NativeID <> bridge id mapping to find their internal ID and use it to
delete the specific file on their side.

For now this is implemented for slack to discord but this will be add to
other bridges where useful.

1 files changed, 9 insertions, 7 deletions

diff --git a/bridge/config/config.go b/bridge/config/config.go
index 7b7c2f2e..16b397f3 100644
--- a/bridge/config/config.go
+++ b/bridge/config/config.go
@@ -23,6 +23,7 @@ const (
 	EventRejoinChannels    = "rejoin_channels"
 	EventUserAction        = "user_action"
 	EventMsgDelete         = "msg_delete"
+	EventFileDelete        = "file_delete"
 	EventAPIConnected      = "api_connected"
 	EventUserTyping        = "user_typing"
 	EventGetChannelMembers = "get_channel_members"
@@ -56,13 +57,14 @@ func (m Message) ParentValid() bool {
 }
 
 type FileInfo struct {
-	Name    string
-	Data    *[]byte
-	Comment string
-	URL     string
-	Size    int64
-	Avatar  bool
-	SHA     string
+	Name     string
+	Data     *[]byte
+	Comment  string
+	URL      string
+	Size     int64
+	Avatar   bool
+	SHA      string
+	NativeID string
 }
 
 type ChannelInfo struct {