It's easy.
First select the "Comment" button on the conversation page, then click on the image icon on the top of the "Add to the conversation" page:
View attachment 780590
A dialog box will open. Click on "From computer" :
View attachment 780591
Another dialog box will appear. Click on the "Choose Files" button:
View attachment 780592
Then select the appropriate file(s) from your directory, click "Open" on the directory button, then click the button "Upload files" when the dialog box opens.
Done. Repeat as necessary