Fix uploading file to thread (#1016)

Author: timja

src/main/java/jenkins/plugins/slack/cache/SlackChannelIdCache.java

@@ -73,11 +73,10 @@ public long getRetryInterval() {
     }
 
     public static String getChannelId(String botUserToken, String channel) throws ExecutionException, InterruptedException, AbortException {
-        if (channel.matches("^(C[A-Z0-9]{8}|G[A-Z0-9]{10}||D[A-Z0-9]{8})$")) {
-            return channel;
-        }
-
         String channelName = cleanChannelName(channel);
+        if (channelName.matches("^(C[A-Z0-9]{8}|C[A-Z0-9]{10}|G[A-Z0-9]{10}|D[A-Z0-9]{8})$")) {
+            return channelName;
+        }
 
         Map<String, String> channelNameToIdMap = CHANNEL_METADATA_CACHE.get(botUserToken);
         String channelId = channelNameToIdMap.get(channelName);