Refactor file extensions to make it a property

This ensures that a refresh picks up the correct files, and avoids having
`extension=` arguments all over the place.

ccdproc/image_collection.py

@@ -108,11 +108,22 @@ def __init__(self, location=None, keywords=None,
         else:
             self._location = ''
 
+        # Set file name extensions
+        # Do our best to keep the file extensions immutable
+        if extensions is not None:
+            if isinstance(extensions, str):
+                # Comma at the end to force it to be a tuple
+                self._file_extensions = (extensions,)
+            else:
+                self._file_extensions = tuple(extensions)
+        else:
+            self._file_extensions = tuple(_recognized_fits_file_extensions)
+
         self._find_fits_by_reading = find_fits_by_reading
 
         self._filenames = filenames
         self._files = []
-        self._files = self._get_files(extensions=extensions)
+        self._files = self._get_files()
 
         if self._files == []:
             warnings.warn("no FITS files in the collection.",
@@ -292,11 +303,19 @@ def glob_exclude(self):
     @property
     def ext(self):
         """
-        str or int, The extension from which the header and data will
+        str or int, The FITS extension from which the header and data will
         be read in all files.
         """
         return self._ext
 
+    @property
+    def file_extensions(self):
+        """
+        List of file name extensions to match when populating or refreshing
+        the ``ImageFileCollection``.
+        """
+        return self._file_extensions
+
     def values(self, keyword, unique=False):
         """
         List of values for a keyword.
@@ -437,7 +456,7 @@ def filter(self, **kwd):
         return ImageFileCollection(filenames=files,
                                    keywords=self.keywords)
 
-    def _get_files(self, extensions=None):
+    def _get_files(self):
         """ Helper method which checks whether ``files`` should be set
         to a subset of file names or to all file names in a directory.
 
@@ -752,18 +771,13 @@ def _find_keywords_by_values(self, **kwd):
         self.summary['file'].mask = ma.nomask
         self.summary['file'].mask[~matches] = True
 
-    def _fits_files_in_directory(self, extensions=None,
+    def _fits_files_in_directory(self,
                                  compressed=True):
         """
         Get names of FITS files in directory, based on filename extension.
 
         Parameters
         ----------
-        extensions : list of str or None, optional
-            List of filename extensions that are FITS files. Default is
-            ``['fit', 'fits', 'fts']``.
-            Default is ``None``.
-
         compressed : bool, optional
             If ``True``, compressed files should be included in the list
             (e.g. `.fits.gz`).
@@ -774,10 +788,12 @@ def _fits_files_in_directory(self, extensions=None,
         list
             *Names* of the files (with extension), not the full pathname.
         """
+        # Force a copy of the extensions to avoid endless combinations of
+        # compression extensions.
+        full_extensions = list(self.file_extensions)
 
-        full_extensions = extensions or list(_recognized_fits_file_extensions)
-
-        # The common compressed fits image .fz is supported using ext=1 when calling ImageFileCollection
+        # The common compressed fits image .fz is supported using ext=1 when
+        # calling ImageFileCollection
         if compressed:
             for comp in ['.gz', '.bz2', '.Z', '.zip', '.fz']:
                 with_comp = [extension + comp for extension in full_extensions]