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Fix Power Automate Excel Attachment Invalid Format Error

Maira Mehtab Sep 22, 2026 869 views

Question details

An automated email flow successfully sends Excel attachments from a personal mailbox, but when sent from a service account, the Excel files become corrupted and cannot be opened.

Product: Excel
Device & OS: not provided
Scenario: Sending automated emails with Excel attachments using a service account mailbox via Power Automate.
Observed behavior: Excel blocks the attachment from opening, displaying an error message stating that the file format or extension is invalid.

Before you start

Ensure that the service account used in your Power Automate flow has explicit read permissions for the source Excel file stored in SharePoint or OneDrive before triggering the email.

Solution 1Recommended

Verify Flow Actions and Dynamic Content Mapping

This solution ensures the service account retrieves the actual file binary rather than just file metadata, which is the most common cause of corrupted email attachments.

When switching from a personal mailbox to a service account, connection references often need to be updated. If the 'Get file content' action fails or is bypassed due to permissions, the 'Send an email' action might send an empty or incorrectly formatted file.

Update Connection References

Open your Power Automate flow, click the three dots on both the 'Get file content' and 'Send an email' actions, and ensure the active connection is authenticated using the service account.

Check SharePoint Permissions

Navigate to the SharePoint document library where the Excel file resides and verify that the service account has at least 'Read' access.

Map File Content Correctly

In the 'Send an email' action, go to the 'Attachments Content' field. Ensure you select the 'File Content' dynamic variable from the previous 'Get file content' step, rather than inputting the file path or item metadata.

Corrupted File Symptoms: If you open the corrupted Excel file in a standard text editor like Notepad and see JSON text or HTML error codes instead of garbled binary data, it confirms that Power Automate passed metadata or an error message instead of the actual file.

Solution 2

Consult the Microsoft Power Platform Community

Since advanced service-account routing and SharePoint file creation fall outside standard Microsoft 365 support, escalating the issue to platform specialists is recommended.

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Frequently Asked Questions

Why does Excel say 'invalid file format or extension' for email attachments?

This error typically occurs when the file is corrupted during transmission, the file extension does not match its actual underlying format, or an automated flow (like Power Automate) attached text metadata instead of the actual binary file content.

Can I use Power Automate to send emails from a service or shared mailbox?

Yes. You must use the 'Send an email from a shared mailbox' action in Power Automate and ensure the account running the flow connection has 'Send As' or 'Send on Behalf' permissions configured in Exchange for that mailbox.

How do I properly attach a SharePoint file to an email in Power Automate?

First, add the 'Get file content' action and point it to your SharePoint file. Then, in your 'Send an email' action, populate the 'Attachments Name' field with the file name (including the .xlsx extension) and map the 'Attachments Content' field specifically to the 'File Content' dynamic variable.

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