Unfortunately, you cannot attach additional files. You can, however, upload your files on a third-party server such as Google Drive or Pitch, and then include the link to it in your cover letter.
📔 In general, it is not recommended that you include a link to your resume in the proposal. Typically, an Upwork profile includes job experiences, and clients who want to view this information can do so by going to the freelancer’s Upwork profile.
Your proposal should typically specify how you can help the client and if you have any previous experience working on similar tasks.