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.
Forcing Attachment Download
For files being opened in the browser directly, you can force download by either clicking on the Download link in the preview window or by right-clicking the attachment file name in the message view and then choosing “Save Link As...” from the browser’s context menu.
If enabled in the Preferences images attached to the message are listed as previews (aka “thumbnails”) right below the message text. Each image has links right next to it which let you either open the image in full size (Show) or Download it to your computer’s hard drive.