Excel error message: “File format or extension invalid”

🧩 Why Excel files from Power Automate sometimes fail to open – and how to fix it

❌ The problem: Excel reports “File format or file extension is invalid”

When saving email attachments to SharePoint via Power Automate, the following error message may appear when opening the file in Excel:

"This workbook could not be opened because the file format may not match the file extension."

This message indicates that the file is corrupted or the content does not match the expected format.

🧠 Cause: Incorrect use of contentBytes from the email trigger

In many cases, the flow saves the attachment immediately after receiving the email using the "Create file" action, using the contentBytes from the email trigger as the file contents. However, this results in the file being saved incompletely or corrupted because the contentBytes do not contain the complete file contents.

A user reports in the Power Platform Community Forum:

"I get an error when opening an Excel file that I saved from Outlook to a document library using Flow. The file won't open." Microsoft Power Platform Community

✅ The solution: Use the “Get attachment (V2)” action

Instead of using contentBytes directly from the email trigger, the Get attachment (V2) action should be used. This action allows you to retrieve the full content of the attachment, which can then be correctly stored in SharePoint.

A community expert explains:

"Please add a 'Get Attachment (V2)' action to get the file content bytes and then use it to create the file." Microsoft Power Platform Community

🔍 Background: Why contentBytes is not enough

The contentBytes property in the email trigger doesn't always contain the full file content, especially for larger or more complex files. This can result in saved files that are only a few bytes in size and cause errors when opened.

An example from the Microsoft Q&A community shows that files were saved with only 4 bytes, which is due to incomplete content bytes. Microsoft Learn

📌 Summary

Procedure	Result
Using contentBytes from the trigger	❌ Errors in Excel
Using “Retrieve Attachment (V2)”	✅ File opens correctly

🚀 Conclusion

When saving email attachments in Power Automate, it's critical to correctly retrieve the complete file content. Using the "Get Attachment (V2)" action ensures that files are saved completely and in the correct format, preventing errors when opening them.

Further information and discussions on this topic can be found in the following sources: