Better media recognation

[Serraniel]

DML currently only can detect real attachments send via discord. This should be extended to checking header of posted links and determining their content type so images can be downloaded, too, if only a direct lnik is posted.

Additionally a list of known hosters should be created which embed the image into the preview like twitter does. As other sites may add their logo to that preview, the list of known hosters is reasonable.