eg. downloads/somenifti.nii.gz?ticket=<ticket>

- attach=1 indicates to initiate download, else stream file contents
with appropriate content-type header

- changes 'engine_request' to 'drone_request'

- related to fixing multipart uploads

- attachment must have unique filename+extention
- attachments can have same (type, kind, state)

- instead of string 'flavor'
- dict() syntax does not allow using variable as keys, {} does

- works on #15

- works on #15

- reduce duplicated code