Make mime type guessing functions more reader-friendly

Leaving an empty "else" execution path, requiring the reader to know
that the default return value is None, seems needlessly confusing.

peekaboo/toolbox/files.py

@@ -37,8 +37,10 @@
 def guess_mime_type_from_file_contents(file_path):
     """  Get type from file magic bytes. """
     mt = magic.from_file(file_path, mime=True)
-    if mt:
-        return mt
+    if not mt:
+        return None
+
+    return mt
 
 
 def guess_mime_type_from_filename(file_path):
@@ -48,5 +50,7 @@ def guess_mime_type_from_filename(file_path):
         mimetypes.add_type('application/javascript', '.jse')
 
     mt = mimetypes.guess_type(file_path)[0]
-    if mt:
-        return mt
+    if not mt:
+        return None
+
+    return mt