If the opened message has any files attached, these are listed on the right, next to the message text. Click on the file name to open or download it. If the file can be displayed directly in the web browser, a preview window will be opened showing the attachment. This will happen for images, text files and PDFs if your browser indicates being able to display them.
Otherwise clicking the file name will initiate the download process and a dialog will most likely appear, asking you where to save the file. If that doesn’t happen, check the default “Downloads” folder on your computer for the attachment file.