Bug: binary output does not work with RawDataOutput on Python 3.x
# Description

When using the ``RawDataOutput`` parameter in an ``execute`` request for a binary output (image/png) one gets the following exception on Python 3.x:

```
Exception: code: 500, locator: 'utf-8' codec can't decode byte 0x89 in position 0: invalid start byte, description: 
Traceback (most recent call last):
  File "/Users/pingu/sandbox/birdhouse/pywps/pywps/app/Service.py", line 623, in __call__
    request_uuid
  File "/Users/pingu/sandbox/birdhouse/pywps/pywps/app/Service.py", line 309, in execute
    response = self._parse_and_execute(process, wps_request, uuid)
  File "/Users/pingu/sandbox/birdhouse/pywps/pywps/app/Service.py", line 388, in _parse_and_execute
    resp = Response(proc_outpt.data)
  File "/Users/pingu/sandbox/birdhouse/pywps/pywps/inout/basic.py", line 199, in get_data
    content = file_handler.read()
  File "/Users/pingu/.conda/envs/workshop3/lib/python3.6/codecs.py", line 321, in decode
    (result, consumed) = self._buffer_decode(data, self.errors, final)
UnicodeDecodeError: 'utf-8' codec can't decode byte 0x89 in position 0: invalid start byte
```

# Environment

- Python version:
Python 3.x
- PyWPS version:
4.0.0, develop branch
- source/distribution
 - [x] git clone
 
# Steps to Reproduce

Run an execute request with ``RawDataOutput=image_output`` for a binary output.

# Additional Information

See also issue #301

I'm working on a bug-fix.

diff --git a/pywps/inout/basic.py b/pywps/inout/basic.py
index 38e7828..1665092 100644
--- a/pywps/inout/basic.py
+++ b/pywps/inout/basic.py
@@ -26,6 +26,21 @@ _SOURCE_TYPE = namedtuple('SOURCE_TYPE', 'MEMORY, FILE, STREAM, DATA')
 SOURCE_TYPE = _SOURCE_TYPE(0, 1, 2, 3)
 
 
+def _is_textfile(filename):
+    try:
+        # use python-magic if available
+        import magic
+        is_text = 'text/' in magic.from_file(filename, mime=True)
+    except ImportError:
+        # read the first part of the file to check for a binary indicator.
+        # This method won't detect all binary files.
+        blocksize = 512
+        fh = open(filename, 'rb')
+        is_text = b'\\x00' not in fh.read(blocksize)
+        fh.close()
+    return is_text
+
+
 class IOHandler(object):
     """Basic IO class. Provides functions, to accept input data in file,
     memory object and stream object and give them out in all three types
@@ -186,16 +201,29 @@ class IOHandler(object):
             else:
                 return StringIO(text_type(self.source))
 
+    def _openmode(self):
+        openmode = 'r'
+        if not PY2:
+            # in Python 3 we need to open binary files in binary mode.
+            checked = False
+            if hasattr(self, 'data_format'):
+                if self.data_format.encoding == 'base64':
+                    # binary, when the data is to be encoded to base64
+                    openmode += 'b'
+                    checked = True
+                elif 'text/' in self.data_format.mime_type:
+                    # not binary, when mime_type is 'text/'
+                    checked = True
+            # when we can't guess it from the mime_type, we need to check the file.
+            # mimetypes like application/xml and application/json are text files too.
+            if not checked and not _is_textfile(self.source):
+                openmode += 'b'
+        return openmode
+
     def get_data(self):
         """Get source as simple data object"""
         if self.source_type == SOURCE_TYPE.FILE:
-            openmode = 'r'
-            if (not PY2 and hasattr(self, 'data_format') and
-                    self.data_format.encoding == 'base64'):
-                # on Python 3, when the data is to be encoded to base64, we
-                # need to open the file in binary mode
-                openmode += 'b'
-            file_handler = open(self.source, mode=openmode)
+            file_handler = open(self.source, mode=self._openmode())
             content = file_handler.read()
             file_handler.close()
             return content
diff --git a/tests/test_inout.py b/tests/test_inout.py
index f830710..c206640 100644
--- a/tests/test_inout.py
+++ b/tests/test_inout.py
@@ -14,7 +14,7 @@ from pywps import Format
 from pywps.validator import get_validator
 from pywps import NAMESPACES
 from pywps.inout.basic import IOHandler, SOURCE_TYPE, SimpleHandler, BBoxInput, BBoxOutput, \\
-    ComplexInput, ComplexOutput, LiteralOutput, LiteralInput
+    ComplexInput, ComplexOutput, LiteralOutput, LiteralInput, _is_textfile
 from pywps.inout import BoundingBoxInput as BoundingBoxInputXML
 from pywps.inout.literaltypes import convert, AllowedValue
 from pywps._compat import StringIO, text_type
@@ -24,6 +24,8 @@ from pywps.validator.mode import MODE
 
 from lxml import etree
 
+DATA_DIR = os.path.join(os.path.abspath(os.path.dirname(__file__)), 'data')
+
 
 def get_data_format(mime_type):
     return Format(mime_type=mime_type,
@@ -146,6 +148,14 @@ class IOHandlerTest(unittest.TestCase):
         self.iohandler.stream.close()
         self.assertEqual(self._value, stream_data, 'Stream obtained')
 
+    def test_is_textfile(self):
+        geotiff = os.path.join(DATA_DIR, 'geotiff', 'dem.tiff')
+        self.assertFalse(_is_textfile(geotiff))
+        gml = os.path.join(DATA_DIR, 'gml', 'point.gml')
+        self.assertTrue(_is_textfile(gml))
+        geojson = os.path.join(DATA_DIR, 'json', 'point.geojson')
+        self.assertTrue(_is_textfile(geojson))
+
 
 class ComplexInputTest(unittest.TestCase):
     """ComplexInput test cases"""
['tests/test_inout.py', 'pywps/inout/basic.py']