This happens because the "Title of attachment" field expects the file name along with the extension of the file. For eg: if title is specified as Sales Data, your email will contain Sales Data in attchment without any extension. If the title is specified as Sales Data.xlsx, then your email will contains Sales Data excel file which you would be able to open.
Example of a file which cannot be opened directly (without manually downloading and renaming to include a file type extension):
Example of a file which can be opened directly: