add multiple file upload

[thesousou]

This commit fixes #8 , by adding multiple file upload to the form while keeping the capability to send a single file.

[dereckson]

Could be cleaner to move upload code logic for one file into a specific function, and use your for loop to call that function:

for upfile in upfiles:
    // 403 check here or in the method
    upload_file(upfile)