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I cannot open an attachment: the file is reported to be corrupt.

This problem seems to affect only Thunderbird users, but can also affect people using other email applications who have been sent email with an attachment by someone using Thunderbird.

The attachment may be shown as present in the email, but when you try to view or open the attachment, it fails, and the file may be reported as damaged or corrupt.

The problem may be that some servers, including Exchange, sometimes report the wrong size for attachments, and Thunderbird, assuming that size to be correct, truncates the attachment as it downloads it.

If you are a Thunderbird user, try the following as a workaround:

  1. The first step is different for Windows PC and Mac users:

    Windows PC users
    : Open the Applications menu (three-barred button at top right), click on Options..., click on the Advanced icon, then click on the General tab.

    Mac users: Click the Thunderbird menu icon, choose Preferences, then select Advanced.

  2. Click on the Config Editor... button.
  3. A warning message may be shown. If so, click on the I'll be careful... button.
  4. In the Config Editor's Filter: box (it may be labelled Search:), enter the following, exactly as shown:


  5. Double-click on mail.server.default.fetch_by_chunks in the table shown, which will change its Value setting from True to False (double-clicking is a toggle, so that if you double-clicked again, the value would revert to what it was before).
  6. Close the Config Editor window (there is no OK button to click here because changes take immediate effect).
  7. Click OK to close the Options window.

This change causes Thunderbird to ignore the incorrect attachment size value and to download the entire attachment.

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This is question number 2334, which appears in the following categories:

Created by Andy Clews on 10 June 2011 and last updated by Andy Clews on 30 July 2015