Search for email containg attachment of specific file type

I'd like to search for an email that contains an excel attachment. I'm using outlook.

I know I can do

hasattachment:true

but this matches any attachment type

I've also tried,

attachment:*.xlsx

but this doesn't work

It's a little backwards, but you can do this with

xlsx hasattachment:true

I just tried this and it works well:

ext:xlsx

Try this - it's just slightly different than what you tried and it works for me. Via the documentation.

attachments:.xlsx

You can search for emails with (for example) Excel spreadsheet attachments by using the following:

filename:xlsx

This is documented in the Advanced Search page.