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).

server.py

@@ -8,16 +8,12 @@ import mimetypes
 import os
 import random
 import shutil
-import signal
-import sleekxmpp
 import ssl
 import string
-import sys
 import yaml
 
 from sleekxmpp.componentxmpp import ComponentXMPP
 from threading import Lock
-from threading import Thread
 
 try:
     # Python 3
@@ -150,6 +146,7 @@ class HttpHandler(BaseHTTPRequestHandler):
                     if mime is None:
                         mime = 'application/octet-stream'
                     self.send_header("Content-Type", mime)
+                    if mime[:6] != 'image/':
                         self.send_header("Content-Disposition", 'attachment; filename="{}"'.format(os.path.basename(filename)))
                     fs = os.fstat(f.fileno())
                     self.send_header("Content-Length", str(fs.st_size))