should not change encoding if specified

__init__.py

@@ -45,7 +45,7 @@ CHARDET_MIN_SIZE = 20
 CHARDET_CONFIDENCE_THRESHOLD = 0.75
 
 def is_text_mimetype(mimetype):
-    return (mimetype.startswith('text/') or mimetype in TEXT_MIMETYPES)    
+    return (mimetype.startswith('text/') or mimetype in TEXT_MIMETYPES)
 
 def guess_encoding(buffer, fallbackencoding=None):
     """try to guess encoding from a buffer"""
@@ -72,13 +72,14 @@ def guess_mimetype_and_encoding(format=None, encoding=None, data=None,
     if filename and not format:
         format, enc = mimetypes.guess_type(filename)
         if format:
-            encoding = enc
+            if not encoding:
+                encoding = enc
         elif enc:
             format = u'application/%s' % enc
         else:
             format = u'application/octet-stream'
     if not encoding and data and format and is_text_mimetype(format):
-        encoding = guess_encoding(data, fallbackencoding)    
+        encoding = guess_encoding(data, fallbackencoding)
     return format, encoding
 
 def html_escape(data):
@@ -111,7 +112,7 @@ class TransformData(object):
     def get(self, attr, default=None):
         """get an optional data attribute"""
         return getattr(self, attr, default)
-    
+
     def decode(self, force=False):
         """return the data as an unicode string"""
         if isinstance(self.data, unicode):
@@ -159,10 +160,10 @@ class TransformData(object):
             self.data = base64.decodestring(self.data)
             self.encoding = guess_encoding(self.data)
 
-            
+
 class MtConverterError(Exception):
     """base class for this package's errors"""
-    
+
 class MissingBinary(MtConverterError):
     """raised when a system binary on whic rely a transform has not been found
     """