file names without extension don't work, gives 404 when tryping to open link even though upload seems to be successful
invalid filenames which get cleaned by "secure_filename" cause an internal server error, possibly related to "t’s your responsibility to ensure that the filename is unique and that you generate random filename if the function returned an empty one." http://werkzeug.pocoo.org/docs/0.9/utils/#werkzeug.utils.secure_filename
Also, if a file with a non-conform filename has been uploaded once and you rename it, it will get uploaded under a different name, but still gives 404.
Found a flaw with "non-conform" file names