Handle uploading multiple files

Tornado, Flask, Django, and most current web frameworks allows for convenient functions for uploading files to the backend. Klein should handle file uploads in a similar fashion as well. Here's a quick example of how it could work:

@route('/')
def example(request):
    # example Request.files
    file_0 = request.files['my_files'][0]
    file_1 = request.files['my_files'][1]

    # example of what a "HTTP File" object would contain
    file_0.filename  # str
    file_0.content    # BytesIO

This issue cannot be addressed without bringing up the infamous Issue 288. There are some great ideas in the comments of that issue. One comment suggests processing args after the request object has been created. Building upon that, we should be able to implement a solution for files (at one point when tx.web2 was a thing, it seems that this was a possible). This would involve extending web.Request or creating a Klein specific Request component. I'm just throwing this idea up here in case anyone has suggestions or has anything.

As a contingency plan, another utility function could be created and it could replicate, for instance, what Tornado does for files.

from klein.utils import fileinfo
my_file = fileinfo(request.args[b'my_file'][0])
print( my_file.filename )
print( my_file.content )

References

• https://stackoverflow.com/questions/17350733/cant-get-uploaded-file-in-twisted
• https://stackoverflow.com/questions/47440098/twisted-upload-muliple-files-key-error

Start with the linked Twisted Issue 288 and the existing web.Request handling; the issue does not name specific Klein files or tests. Compare the proposed request.files interface with the alternative klein.utils.fileinfo approach and the referenced framework behavior. Done means Klein has a decided, tested way to expose multiple uploaded files with filename and content.