Set a default filename so that Slack displays the file preview correctly

CHANGELOG.md

@@ -1,6 +1,7 @@
 ### 3.3.0 (Next)
 
 * [#602](https://github.com/slack-ruby/slack-ruby-client/pull/602): Make gli an optional dependency; only the `slack` command-line client needs it - [@corsonknowles](https://github.com/corsonknowles).
+* [#599](https://github.com/slack-ruby/slack-ruby-client/pull/599): Set a default filename in `files_upload` so Slack displays image previews correctly when none is specified - [@ts-3156](https://github.com/ts-3156), [@dblock](https://github.com/dblock).
 * [#591](https://github.com/slack-ruby/slack-ruby-client/pull/591): Generate AI CHANGELOG entries and PR summaries for automated API update PRs, lock simplecov below 1.1.0 to avoid breaking Coveralls - [@dblock](https://github.com/dblock).
 * [#590](https://github.com/slack-ruby/slack-ruby-client/pull/590): Add entity.acknowledgeCommentAction and entity.presentComments methods - [@slack-ruby-ci-bot](https://github.com/apps/slack-ruby-ci-bot).
 * [#590](https://github.com/slack-ruby/slack-ruby-client/pull/590): Add admin.apps.mcpServers, admin.apps.mcpServers.permissions, and admin.apps.permissions endpoints - [@slack-ruby-ci-bot](https://github.com/apps/slack-ruby-ci-bot).

lib/slack/web/api/endpoints/files.rb

@@ -192,6 +192,7 @@ def files_sharedPublicURL(options = {})
           # @see https://api.slack.com/methods/files.upload
           # @see https://github.com/slack-ruby/slack-api-ref/blob/master/methods/files/files.upload.json
           def files_upload(options = {})
+            options = options.merge(filename: 'file') if options[:file] && !options[:filename]
             post('files.upload', options)
           end
         end

lib/slack/web/api/patches/files.default-filename.patch

@@ -0,0 +1,12 @@
+diff --git a/lib/slack/web/api/endpoints/files.rb b/lib/slack/web/api/endpoints/files.rb
+index 13a1889..9ce4894 100644
+--- a/lib/slack/web/api/endpoints/files.rb
++++ b/lib/slack/web/api/endpoints/files.rb
+@@ -192,6 +192,7 @@ module Slack
+           # @see https://api.slack.com/methods/files.upload
+           # @see https://github.com/slack-ruby/slack-api-ref/blob/master/methods/files/files.upload.json
+           def files_upload(options = {})
++            options = options.merge(filename: 'file') if options[:file] && !options[:filename]
+             post('files.upload', options)
+           end
+         end

spec/slack/web/api/endpoints/custom/files_spec.rb

@@ -4,6 +4,25 @@
 RSpec.describe Slack::Web::Api::Endpoints::Files do
   let(:client) { Slack::Web::Client.new }
 
+  describe '#files_upload' do
+    it 'defaults the filename when a file is supplied' do
+      options = { file: 'file contents' }
+      expect(client).to receive(:post).with('files.upload', { file: 'file contents', filename: 'file' })
+      client.files_upload(options)
+      expect(options).to eq(file: 'file contents')
+    end
+
+    it 'preserves an explicit filename' do
+      expect(client).to receive(:post).with('files.upload', { file: 'file contents', filename: 'image.png' })
+      client.files_upload(file: 'file contents', filename: 'image.png')
+    end
+
+    it 'does not add a filename when no file is supplied' do
+      expect(client).to receive(:post).with('files.upload', { content: 'text' })
+      client.files_upload(content: 'text')
+    end
+  end
+
   %w[filename content].each do |arg|
     context "when #{arg} is missing from options" do
       let(:params) do