Make output_mimetype accessible from the class.

Traitlets are only accessible by instantiating the class. There's no
clear reason that this needs to be a traitlet, anyway.

IPython/nbconvert/exporters/exporter.py

@@ -53,9 +53,10 b' class Exporter(LoggingConfigurable):'
         help="Extension of the file that should be written to disk"
         )
 
-    output_mimetype = Unicode('', config=True,
-        help="MIME type of the result file, for HTTP response headers."
-        )
+    # MIME type of the result file, for HTTP response headers.
+    # This is *not* a traitlet, because we want to be able to access it from
+    # the class, not just on instances.
+    output_mimetype = ''
 
     #Configurability, allows the user to easily add filters and preprocessors.
     preprocessors = List(config=True,

IPython/nbconvert/exporters/html.py

@@ -43,8 +43,7 b' class HTMLExporter(TemplateExporter):'
     default_template = Unicode('full', config=True, help="""Flavor of the data 
         format to use.  I.E. 'full' or 'basic'""")
     
-    def _output_mimetype_default(self):
-        return 'text/html'
+    output_mimetype = 'text/html'
     
     @property
     def default_config(self):

IPython/nbconvert/exporters/latex.py

@@ -63,8 +63,7 b' class LatexExporter(TemplateExporter):'
     #Extension that the template files use.    
     template_extension = Unicode(".tplx", config=True)
 
-    def _output_mimetype_default(self):
-        return 'text/latex'
+    output_mimetype = 'text/latex'
 
 
     @property

IPython/nbconvert/exporters/markdown.py

@@ -30,8 +30,7 b' class MarkdownExporter(TemplateExporter):'
         'md', config=True, 
         help="Extension of the file that should be written to disk")
 
-    def _output_mimetype_default(self):
-        return 'text/markdown'
+    output_mimetype = 'text/markdown'
     
     def _raw_mimetypes_default(self):
         return ['text/markdown', 'text/html', '']

IPython/nbconvert/exporters/python.py

@@ -29,5 +29,4 b' class PythonExporter(TemplateExporter):'
         'py', config=True, 
         help="Extension of the file that should be written to disk")
 
-    def _output_mimetype_default(self):
-        return 'text/x-python'
+    output_mimetype = 'text/x-python'

IPython/nbconvert/exporters/rst.py

@@ -30,8 +30,7 b' class RSTExporter(TemplateExporter):'
         'rst', config=True, 
         help="Extension of the file that should be written to disk")
 
-    def _output_mimetype_default(self):
-        return 'text/restructuredtext'
+    output_mimetype = 'text/restructuredtext'
 
     @property
     def default_config(self):

IPython/nbconvert/exporters/slides.py

@@ -31,8 +31,7 b' class SlidesExporter(HTMLExporter):'
         help="Extension of the file that should be written to disk"
         )
 
-    def _output_mimetype_default(self):
-        return 'text/html'
+    output_mimetype = 'text/html'
 
     default_template = Unicode('reveal', config=True, help="""Template of the 
         data format to use.  I.E. 'reveal'""")

IPython/nbconvert/exporters/templateexporter.py

@@ -126,8 +126,6 b' class TemplateExporter(Exporter):'
         help="""Dictionary of filters, by name and namespace, to add to the Jinja
         environment.""")
 
-    output_mimetype = Unicode('')
-
     raw_mimetypes = List(config=True,
         help="""formats of raw cells to be included in this Exporter's output."""
     )