You may be able to, but only if your administrator has set up a field that supports file attachments on your timesheet, time off booking, or project. If they have, the field will be located:
Attaching files can be useful if your timesheet or time off booking requires supporting documentation – say, a sick note if you’re booking sick leave, or a signed statement of work for a project.
If an attachment field is available, you’ll be able to enter a document URL or upload a document, or you may be able to choose between these two methods. If you have the option of uploading a file, you can drag and drop it. Attachment fields support a broad range of file formats.