re: "It is possible to upload in a conversation with someone but not in job description. Providing external link it is not always possible."
You are able to use the Upwork Messenger tool to upload many types of files within a private conversation - a room which is automatically created for a freelancer to speak with a client/potential client.
Many types of files (but not all types) may indeed be uploaded to a conversation/room.
You are also welcome to use any form of communications you want to use when communicating or sharing filse with freelancers who you are interviewing or working with. You may us email, Skype, Dropbox, Google Drive, Microsoft One Drive, USB flashdrive, FTP, WeTransfer, ExaVault, etc.