TL;DR: Use the expression to find if file is an .xlsx : endsWith(triggerOutputs()?['headers/x-ms-file-id'],'.xlsx')
Details:
Step 1 : Automated Flow (when a new file is created in SharePoint Folder)
Step 2 : Condition using expression listed above
Step 3 : if false, send an email saying wrong file type
Step 4 : if true, do nothing
Under this method, you will be wasting your daily Flow runs allowance as whenever an .xlsx is uploaded the Flow will run and do nothing. To mitigate this, you can add a trigger condiiton to your flow.
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