Normaly excel files are not defined to xenforo to upload as resource but you can do it easly as below:
First Logind to Xenforo Admin Panel and navigate to Admin>Setup>Options>Attachments

Then you can see that excel files are not alowed with current settings:

You can add xlsx excel files manually as below:

Then click to save button.
You will be able to upload excel files to xenforo.
First Logind to Xenforo Admin Panel and navigate to Admin>Setup>Options>Attachments

Then you can see that excel files are not alowed with current settings:

You can add xlsx excel files manually as below:

Then click to save button.
You will be able to upload excel files to xenforo.