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Adding remote files to Slack

The rundown
Read this if:You're still experimenting, prototyping, and exploring.
Read first:Uploading files to Slack
Read next:An overview of message interactivity

Remote files are files not hosted inside Slack. Think of a remote file as a pointer, a forwarding address, a reference to a file that lives elsewhere.

For most apps, uploading files directly to Slack is preferred over adding remote files. Direct uploads are simpler, like an easy button for files.

However, there are a few benefits to adding and sharing Remote Files to Slack.

  1. If your file can't be hosted in Slack for property rights reasons, Remote Files are here to save the day.
  2. If you wish to provide a lush, custom unfurl experience for your files when their links are shared, Remote Files bring forth fantastical unfurls for the low low price of a JSON object.
  3. If you want to control the way your file appears in Slack searches, so that others can find it later, Remote Files allow for customized indexable content. That way, you can make sure your file appears when users search for certain terms.

You can add, update, remove, share, and unfurl remote files in Slack.

Why use remote files?

Imagine an app that shares your personal travel writing to a Slack channel once a week. Your latest story is stored in a private file service as a document, containing both text and images.

Your app could simply write a message in channel containing a link to your latest story. You'd have no control over the unfurl, and the doc wouldn't appear in Slack searches. If you don't have an existing app installed that beautifies your link, you'll get no unfurl at all when sharing the link—which means all the lovely context and metadata from the story wouldn't be displayed.

Here's what a simple URL share would look like:

a doc link shared in a Slack channel

With remote files, we can tidy up the message, removing the long and unneeded URL, and we can choose a tempting preview image to encourage people to read on. To do this, we're going to share the story as a remote file.

Adding remote files

To add a remote file, use the files.remote.add method from the Web API.

Remote files exist across the whole workspace (or organization, for Enterprise Grid). Because of that, remote files must be added by bots with the bot scope, not by an individual user.

Here's a sample call:

curl -F token=<bot_user_token> \
    -F external_id=ABCD1 \
    -F external_url=https://mydocuments.com/document/d/1TA9fIaph4eSz2fC_1JGMuYaYUc4IvieIop0WqfCXw5Y/edit?usp=sharing \
    -F title=LeadvilleAndBackAgain \
    -F preview_image=@cycling.jpg  \
    -F indexable_file_contents=search_terms.txt
    https://slack.com/api/files.remote.add

Pay particular attention to the preview_image parameter. preview_image should be a binary image file, and it will be stored in Slack. That's going to allow a more beautiful unfurl in the next step, when the app actually shares the remote file.

indexable_file_contents also deserves a mention. Use this parameter to specify a file containing the search terms that correspond to this remote file.

Once your upload succeeds, you'll see an HTTP response from Slack containing an "ok": true field, plus a file object. For more detail on file objects and the fields contained inside, a look at the file object documentation is highly recommended.

You'll see the external_id returned to you on the file object in the Slack response, as well as a file_id. Either of these ids can be stored and used to query the files.remote.info method to access other information about the file.

Sharing remote files

Adding is nice, but sharing is caring.

In other words, adding a remote file does not, by itself, share the file to a conversation. Without sharing the file, it remains an orphan inside Slack—you can view information on it via files.remote.info method, but it won't actually be visible anywhere. Let's make it visible with a call to the files.remote.share method.

curl -F token=<bot_user_token> \
    -F external_id=ABCD1 \
    -F channels=C12345 \
    https://slack.com/api/files.remote.share

Now we get a lovely message in channel that shows off the remote file (in this case, a story about cycling) as it should be seen.

A remote file upload

Now we have a custom preview_image, which we specified during the add step, instead of the ugly raw URL and a small picture of text.

A word about tokens: the files.remote.share method may be called with either a bot or a user token. The bot token shares the file from your app, while the user token shares the file from the user associated with your user token. Use the bot token to share to channels that the bot has access to; use the user token to share to channels that the user has access to.

Unfurling remote files

We've already mentioned how to add a custom preview_image that makes our remote files appear nicer when they're shared. But what about when someone else shares our remote file as a link?

As explained in our app setup guide, the first step to providing lush unfurls is to be armed with the link_shared event.

When that link_shared event fires, we receive its payload, including the message_ts of the message that triggered it, as well as the channel of that message. Now we can make an HTTP request to chat.unfurl. First, let's set up our unfurls object using message blocks:

unfurls: {
        'https://mydocuments.com/document/d/1TA9fIaph4eSz2fC_1JGMuYaYUc4IvieIop0WqfCXw5Y/edit?usp=sharing': {
          hide_color: true,
          blocks: [{
            type: 'file',
            external_id: 'ABCD1',
            source: 'remote',
          }]
        }
      }

Next we call the chat.unfurl method with the channel from the link_shared event, and set the ts equal to the message_ts from the event:

curl -F token=<user_token> \
    -F ts=123456789.9875 \
    -F unfurls=<unfurls json from above> \
    -F channel=C12345 \
    -F user_auth_required=false \
    https://slack.com/api/chat.unfurl

Check out the chat.unfurl method documentation for more detail on how to use that method to its fullest potential.

Updating remote files

Your remote file's contents may change. When that happens, call the files.remote.update method to update your remote file:

curl -F token=<bot_user_token> \
    -F tile=ACyclistTale \
    -F external_id=ABCD1 \
    https://slack.com/api/files.remote.update

You cannot update the external_id or file_id of a remote file. If you need to change those fields, your best bet is to remove and then add the file.

One other piece of pleasant news: adding a remote file is actually an "upsert" operation, meaning that if you add a file that has been added before, the existing file will be updated.

Removing remote files

Removing a remote file follows the same pattern as adding and updating. Use the files.remote.remove method to remove a remote file from Slack.

This method does not delete the remote file from where it's externally hosted; it only removes the remote file from Slack.

    curl -F token=<bot_token> \
    -F external_id=ABCD1 \
    https://slack.com/api/files.remote.remove

After removal, any place that used to display the file will show a tombstone message containing the text "This file was deleted." instead.

Epilogue to files

That about wraps up our guide to remote files. Hopefully, they now feel less like a free-for-all and more like a fun, functional way to add richer context to Slack. Although direct uploads are always preferred to remote files for their simplicity, remote files do allow for lush unfurls, custom searchable contents, and private hosting of files.