Upload multiple files to repo via web-interface
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### Updated by @blackst0ne (2017-05-30)
#### Design
##### Illustrations
1. [illustration-uploading-files.svg](/uploads/ec475716cdeadf99ec0e691af3334776/illustration-uploading-files.svg)
2. [illustration-uploading-files-failed.svg](https://gitlab.com/gitlab-org/gitlab-ce/uploads/578d635303836fabe0b9aa7a5e1b0923/illustration-uploading-files-failed.svg)
##### Dropdown
- Use `Upload "files"` directly as `Upload files` means you can upload a single file and multiple files.

##### Upload files modal - no files
- Illustration: [illustration-uploading-files.svg](/uploads/1e3c2dd3e6f06e8037f5c9103e171ddc/illustration-uploading-files.svg)

##### Upload files modal - multiple files
- If the file name is very long, the file name will be truncated.
* Show the first and last text of the name and the extension name.
- After clicking "Close", the files in the modal should be removed.
- Add "Clean queue" button.
- All types of files should have to show the images. Currently, text files show nothing, but the image files show the image. That makes the screen look a little messy.
* text file --> `fa-file-text-o` icon from Font Awesome.

##### Uploading error

---
### Original proposal
in the `Files` section of a project, i can upload single files (selecting the <kbd>+</kbd> symbol and then choosing `Upload file` in the pull-down menu).
this gives me a drag&drop panel where i can drop a single file.
however, when trying to add more than one file, i get:
> Error uploading file: "You can not upload any more files."

I find this a bit disappointing (as there is no problem to add multiple files in one commit when doing it via git).
tested on `gitlab-ce omnibus 8.4.3` and confirmed today on `gitlab.com`
issue