Download single file from BitBucket server

Navigate to your repo and copy link location of your Raw File option.
Download the file

curl --silent --show-error --user <USERNAME> https://example.com/projects/<PROJNAME>/repos/<YOURREPO>/raw/<FOLDER>/<FILE>?at=refs%2Fheads%2Fmaster -o <OUTPUTFILENAME>

Tested on

Atlassian Bitbucket v7.5.0