Bug#376727:

[Miguel Quirós]

I have found the same behaviour in files with extensions jpg. If the
files are renamed with the extension jpeg, then they are treated
correctly. It seems that the extension jpg is not considered valid for a
file of JPEG type whereas the extension jpeg is considered OK.

Nevertheless, in the file /etc/mime.types (it is where the association
between filename extensions and mime types are defined, isn't it?), the
jpeg line seems correct:

image/jpeg          jpeg jpg jpe

I have not tested png files.
-- 
Miguel Quirós Olozábal
Departamento de Química Inorgánica. Facultad de Ciencias.
Universidad de Granada. 18071 Granada. SPAIN
email:mquiros<at>ugr<dot>es
      mquiros<arroba>ugr<punto>es