Allow user file upload to work (#967)

Author: timja

README.md

@@ -143,6 +143,18 @@ node {
 
 This feature requires [botUser](#bot-user-mode) mode.
 
+##### File upload to a user channel
+
+You can upload files to a user channel by messaging the user first and then using the channel ID from the message response:
+
+```groovy
+node {
+  sh "echo hey > blah.txt"
+  def slackResponse = slackSend channel: '$userId', message: 'Hey', sendAsText: true
+  slackUploadFile filePath: "*.txt", channel: slackResponse.channelId
+}
+```
+
 #### Threads Support
 
 You can send a message and create a thread on that message using the pipeline step.

src/main/java/jenkins/plugins/slack/pipeline/SlackUploadFileStep.java

@@ -141,15 +141,11 @@ protected Void run() throws IOException, InterruptedException, ExecutionExceptio
 
             String channelId;
             try {
-                channelId = SlackChannelIdCache.getChannelId(populatedToken, cleanChannelName(channel));
+                String channelName = cleanChannelName(channel);
+                channelId = SlackChannelIdCache.getChannelId(populatedToken, channelName);
                 if (channelId == null) {
-                    String message = "Failed uploading file to slack, channel not found: " + channel;
-                    if (step.failOnError) {
-                        throw new AbortException(message);
-                    } else {
-                        listener.error(message);
-                        return null;
-                    }
+                    // possibly a user ID which won't be found in the channel ID cache
+                    channelId = channelName;
                 }
             } catch (CompletionException | SlackChannelIdCache.HttpStatusCodeException e) {
                 throw new AbortException("Failed uploading file to slack, channel not found: " + channel + ", error: " + e.getMessage());