Filelink for Large Attachments

Many email servers do not accept messages with large file attachments. The file size restriction varies depending on the mail server configuration. While you might be able to send a message with a large attachment, the receiving mail server might refuse to accept a message with a large attachment. This is all beyond the control of Thunderbird.

Thunderbird Filelink eliminates this problem by providing support for online storage services. It allows you to upload attachments to an online storage service and then replaces the attachment in the message with a link. The message recipient clicks the link to download the attachment. As additional benefits, sending and receiving large files is much faster and both you and the recipient will save disk space.

Note that you can use Thunderbird Filelink in addition to conventional attachments. For example, you can attach small files directly to a message and use Filelink for large attachments in the same message.

The instructions below show how to configure and use Filelink with the Box online storage provider. We have an agreement with them to integrate their services into Thunderbird as well. (We used to have an agreement with Hightail, formerly YouSendIt). However, the community has been active in adding other providers through the use of add-ons with FileRun, hubiC, Mega, ownCloud-based services, Pydio (formerly AjaXplorer), Synnefo, and ~okeanos currently having add-ons.

Configuring Filelink

To use Filelink, you must first create a new account (or configure an existing account) with an online storage service provider.

  1. At the top of the Thunderbird window, click the Tools menu and select OptionsIn the menu bar, click the Thunderbird menu and select PreferencesAt the top of the Thunderbird window, click the Edit menu and select Preferences, or click the menu button New Fx Menufx57menu and choose OptionsPreferences.
  2. Select the Attachments panel and choose the Outgoing tab.
  3. Click the Add button. The Set up Filelink panel will open.
  4. Select the desired service provider from the drop-down list.
    • Choose Box, click Set up Account, or click the Get a Box account… link to create a new account.
      Note: In order to set up a different account, you need to install an add-on for its provider first (see below.)
  5. Click Set up Account to continue.

If you are creating a new account, a browser window will open that displays the service provider's registration page. Create an account as instructed. The service provider will send you a validation email message. Click the validation link in the message to activate your account. Enter your username (your email address) in the dialog shown above and enter your password when prompted.

In the main configuration dialog, check Offer to share for files larger than and specify a file size (in megabytes) if you want Thunderbird to prompt you to use Filelink when a message attachment exceeds the specified size.

Using Filelink

When you attach a file to a message and the file size exceeds the size you have specified above, Thunderbird will automatically offer you the use of Filelink:


To force a file to be attached using Filelink (rather than relying on Thunderbird to determine if the attached file exceeds the specified size), you can click the down arrow next to the Attach button on the message toolbar to access the Filelink menu option.

Click the Link button to upload the file to the online storage service provider. (Alternatively click Ignore to attach the file as a conventional attachment.)

When you select Link the displayed message will update to:


When the linking is complete – that is your attachment has been uploaded to the service provider – your email will be modified and a block of text and a link will be added to the body of your email as shown below:


Note: The message will appear in plain text, if Compose messages in HTML format is turned off in Account Settings.

When the recipients receive the message, they will see the same information and clicking the link in the message will take them to a download page to download the attachment.

Filelink FAQ

Attachments that you send via Filelink are not stored on Mozilla servers. Each file storage service provider has its own privacy policy and terms of service. The file storage service provider has reviewed the Filelink feature to ensure that it is consistent with their policies.

Q: Does Mozilla know which provider the user has picked?

A: No. Mozilla is unaware of which provider has been selected by a user. Provider configuration is stored on the local machine.

Q: What storage services are currently supported?

A: We have reached an agreement with Box to integrate their services directly into Thunderbird. The agreement with Hightail is no longer valid. You can install support for more providers with add-ons, like:

  • Last but not least, you can use Filelink with a WebDAV server with the WebDAV for Filelink add-on.

Q: Can the storage service view my attachments?

A: Unless you encrypt the file before uploading, the storage services will be able to view the file, as will anyone who obtains the link to the attachment. Users must decide on their own which service provider they trust with that responsibility. (Service providers generally explain your privacy rights in their terms of service.) Note that standard attachment functionality is also not encrypted. When you send an attachment the "normal" way, the file can be accessed by anyone that can access the message between your system and the recipients system (such as your email service provider). Filelink adds some security as file uploads to the storage service provider are sent via HTTPS, a secure protocol.

Q: How long will my file be available on the storage service provider's site?

A: The file will remain available on the storage service provider's site until you specifically delete it. Log in to the provider's website to view and delete the files in your storage space.

Q: Can you support service provider X or protocol Y?

A: We plan to support SpiderOak. We also have an "Up-for-grabs" project available if anybody would like to add support for more services or protocols. Also have a look at the Developer documentation for the Filelink feature.

Q: I don't like the idea of storing my files with third-parties like this. Do regular attachments still work?

A: Regular email attachments still work the same as they always have. We've just given you the ability to choose to upload large attachments somewhere else, if you wish. Thunderbird's notion of a large file defaults to 1 MB, but can be customized in Thunderbird's Preferences:

  1. At the top of the Thunderbird window, click the Tools menu and select OptionsIn the menu bar, click the Thunderbird menu and select PreferencesAt the top of the Thunderbird window, click the Edit menu and select Preferences, or click the menu button New Fx Menufx57menu and choose OptionsPreferences.
  2. Select the Attachment panel and choose the Outgoing tab.
  3. Set the file size for the Filelink warning.

The offer to upload can also be disabled entirely by removing the selection from the Offer to share for files larger than option.

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