First step

• A given File upload field can hold more than one file attachment. A user can add another "upload input", part of the same field. A user can also remove an "upload input", while keeping at least one "upload input" per field.
• Selection of files to upload can be multiple
• User can drag 'n drop files
• When uploading a file, it should be split into chunks. The first chunk will be sent using POST /artifact_temporary_files, then the following chunks will be sent using PUT /artifact_temporary_files/:id. The chunks will be appended to the same file.
• When an error occured during an upload, the user can retry uploading.
• A user can remove an uploaded file, which triggers a DELETE /artifact_temporary_files/:id.
• when I click on the modal's "Submit" button, then the list of temporary files' ids will be sent with the PUT /artifacts/:id request. It will bind the files to the artifact.

Second step

• The existing limit (5) on total temporary files should be removed.
• Instead, a new limit should be added: a given user should have a total file size quota that is the sum of all her temporary files' sizes. E.g. with a limit of 64 MB, if userA already has uploaded File1 of 33 MB, userA can upload File2 of 16 MB but cannot upload File3 of 32 MB.
• A new query should be added to the existing REST route GET /artifact_temporary_files. It should respond with the remaining file size quota for the logged user.
• The remaining file size quota for the logged user should be displayed near the file upload field.
• A cron should be added. It will clean up temporary files that are older than (?).

Third step

• An upload progress bar should be displayed. It should fill up as file chunks are uploaded and the server acknowledges.
• Files should be uploaded before hitting the modal's "Submit" button. E.g. When I click on the "Upload" button of an "upload input", uploading starts immediately without waiting for me to hit the modal's "Submit" button.
• When uploading a file, a message that details the remaining % for the ongoing upload(s) is displayed near the modal's "Submit" button. If user clicks on the submit button while files are still uploading, then the button is disabled and is labeled differently.
• When an error occured during at least one upload, a message is displayed near the modal's "Submit" button. Button switchs back to enabled.

Notes

• There can be more than one File upload fields on a given tracker.
• IE9 File upload won't be supported

Possible frontend libraries :

• http://flowjs.github.io/ng-flow/
• https://github.com/danialfarid/ng-file-upload
• Custom solution : see http://jsfiddle.net/43suLw6e/3/