Error uploading file when mime type is undefined and size < 10mb.

[millerlucas]

Rocket.Chat Version: 0.55.1
Running Instances: 1
DB Replicaset OpLog: Disabled
Node Version: v4.8.1

Upload fail when the file type is not recognized by browser and the size is lower than 10 mb.

Uncaught TypeError: Cannot read property 'indexOf' of undefined
    at r (https://chat.teamto.fr/3ddab15a0c8874398ba3db26458f5d286c68a2ef.js?meteor_js_resource=true:260:14377)
    at i (https://chat.teamto.fr/3ddab15a0c8874398ba3db26458f5d286c68a2ef.js?meteor_js_resource=true:260:15133)
    at fileUpload (https://chat.teamto.fr/3ddab15a0c8874398ba3db26458f5d286c68a2ef.js?meteor_js_resource=true:260:18133)
    at Object.dropped .dropzone-overlay (https://chat.teamto.fr/3ddab15a0c8874398ba3db26458f5d286c68a2ef.js?meteor_js_resource=true:264:1461)
    at Object.o.(anonymous function) (https://chat.teamto.fr/3ddab15a0c8874398ba3db26458f5d286c68a2ef.js?meteor_js_resource=true:258:620)
    at https://chat.teamto.fr/3ddab15a0c8874398ba3db26458f5d286c68a2ef.js?meteor_js_resource=true:130:6128
    at Function.e._withTemplateInstanceFunc (https://chat.teamto.fr/3ddab15a0c8874398ba3db26458f5d286c68a2ef.js?meteor_js_resource=true:130:5707)
    at h.View.<anonymous> (https://chat.teamto.fr/3ddab15a0c8874398ba3db26458f5d286c68a2ef.js?meteor_js_resource=true:130:6080)
    at https://chat.teamto.fr/3ddab15a0c8874398ba3db26458f5d286c68a2ef.js?meteor_js_resource=true:129:27045
    at Object.h._withCurrentView (https://chat.teamto.fr/3ddab15a0c8874398ba3db26458f5d286c68a2ef.js?meteor_js_resource=true:129:23116)

The error come from the file rocketchat-ui/client/lib/fileUpload.coffee line 13, file.file.type is undefined.

I think maybe we should avoid trying to preview the file in this case.