Just send Content-disposition: attachment-Header if content is no image
Just send Content-disposition: attachment-Header if content is no image

Content-disposition: attachment prevents most code to be executed in the
browser (Java being an exception), but this is not needed for images and
makes viewing images in clients that don't show the pictures inline even
more complicated.

(Also this PR removes unused imports).

SamWhited/HttpUploadComponent,SamWhited/HttpUploadComponent,siacs/HttpUploadComponent,siacs/HttpUploadComponent