How do I tell roo to open a paperclip attachment after uploaded to later processing?

Hello, I’m new to ruby and rails… I was planning to use the spreadsheet gem to open the files but it doesn’t support xlsx.
I saw this as an example on how to do the same using paperclip

@workbook =
I imagine with roo would be something like
# In this example the model MyFile has_attached_file :attachment
@workbook =
**Like I said I'm new to Ruby and Rails, so I don't know what's "first", I assume it refers to the first record (the lastest) on the MyFile corresponding Table**
Is this the right way to do it? I find the use of "new" confusing, although it says  loads an Excel Spreadsheet for Excel .xlsx files I think it's creating a new one instead of loading the attachment. Thank you for your guidance in advance.