Unknown filetypes are not sent

[simmac]

Files with a type unknown to File.probeContentType() (or URLConnection.guessContentTypeFromName, so this issue is not dependent on #11) can't be sent in E2E-mode.
The two named methods return null for unknown filetypes, the Gateway SDK seems to ignore files with mimeType=null.

I would recommend checking for null when calling the filetype-function and using either MIME-type application/unknown or application/octet-stream for unknown filetypes (and maybe notifying the sender that this file will be sent as unknown via console output).