Add detection mime-types for upload files

Eliminate OS dependence from mime-types. e.g. `foo.csv` is
`text/csv` on Linux, but `application/vnd.ms-excel` on Windows 7.

packages/rocketchat-ui-message/message/messageBox.coffee

 import toastr from 'toastr'
+import mime from 'mime-type/with-db'
+
 katexSyntax = ->
 	if RocketChat.katex.katex_enabled()
 		return "$$KaTeX$$"   if RocketChat.katex.dollar_syntax_enabled()
@@ -192,6 +194,8 @@ Template.messageBox.events
 
 		filesToUpload = []
 		for file in files
+			# `file.type = mime.lookup(file.name)` does not work.
+			Object.defineProperty(file, 'type', { value: mime.lookup(file.name) })
 			filesToUpload.push
 				file: file
 				name: file.name

packages/rocketchat-ui/views/app/room.coffee

 import moment from 'moment'
+import mime from 'mime-type/with-db'
 
 socialSharing = (options = {}) ->
 	window.plugins.socialsharing.share(options.message, options.subject, options.file, options.link)
@@ -407,6 +408,8 @@ Template.room.events
 
 		filesToUpload = []
 		for file in files
+			# `file.type = mime.lookup(file.name)` does not work.
+			Object.defineProperty(file, 'type', { value: mime.lookup(file.name) })
 			filesToUpload.push
 				file: file
 				name: file.name