I have a flow that is supposed to check if an email is received with specific parameters and if it is, save it to OneDrive with a specific name. This is the flow

The apply to each is "Attachments", and the file content in Update File is "Attachment Content". The file I'm updating is an xls file - not xlsx.
When the flow runs, the file content is the word "null" instead of the actual file content. Is this because of using the outdated filetype? If so, how can I get around it? The file is from a third party vendor so I do not control the filetype. I tried to save the content to a file with the xlsx extension, and that resulted in a corrupted file.