Import files with different extension type from the same folder

You can modify your Power Query code to use different file ingestion routines (called functions) depending on the extension type of the file.  Note that .xlsx files have a totally different structure (and potentially many tabs that you need to choose from) than plain .txt files.

Something like this

let
Source = SharePoint.Files("https://xxx.sharepoint.com/teams/yyy", [ApiVersion = 15]),
#"Filtered Rows" = Table.SelectRows(Source, each ([Extension] = ".txt" or [Extension] = ".xlsx")),
#"Get File Contents" = Table.AddColumn(#"Filtered Rows", "File", each if [Extension]=".txt" then GetTXT([Content]) else GetXLSX([Content]))

in 

#"Get File Contents"