diff --git a/sat/plugins/plugin_xep_0054.py b/sat/plugins/plugin_xep_0054.py
index a856fbd0..bacbb183 100644
--- a/sat/plugins/plugin_xep_0054.py
+++ b/sat/plugins/plugin_xep_0054.py
@@ -234,12 +234,13 @@ class XEP_0054(object):
         try:
             mime_type = unicode(photo_elt.elements(NS_VCARD, "TYPE").next())
         except StopIteration:
-            log.warning(u"no MIME type found, assuming image/png")
-            mime_type = u"image/png"
+            mime_type = None
         else:
             if not mime_type:
-                log.warning(u"empty MIME type, assuming image/png")
-                mime_type = u"image/png"
+                # MIME type not know, we'll only support PNG files
+                # TODO: autodetection using e.g. "magic" module
+                #       (https://pypi.org/project/python-magic/)
+                mime_type = None
             elif mime_type not in ("image/gif", "image/jpeg", "image/png"):
                 if mime_type == "image/x-png":
                     # XXX: this old MIME type is still used by some clients
@@ -270,6 +271,15 @@ class XEP_0054(object):
         if not buf:
             log.warning(u"empty avatar for {jid}".format(jid=entity_jid.full()))
             raise Failure(exceptions.NotFound())
+        if mime_type is None:
+            log.warning(_(u"no MIME type found for {entity}'s avatar, assuming image/png")
+                .format(entity=entity_jid.full()))
+            if buf[:8] != b'\x89\x50\x4e\x47\x0d\x0a\x1a\x0a':
+                log.warning(u"this is not a PNG file, ignoring it")
+                raise Failure(exceptions.DataError())
+            else:
+                mime_type = u"image/png"
+
         log.debug(_(u"Decoding binary"))
         decoded = b64decode(buf)
         del buf