Fix stream_to_open_named_file for non-utf8 text

We only recode the file if a target encoding is given,
and make the assumption that the file is not binary
or compressed if a target encoding is set.

This is required to upload the imzml test data by url
in https://github.com/galaxyproject/galaxy/issues/7412#issuecomment-469379295

lib/galaxy/datatypes/sniff.py

@@ -23,10 +23,7 @@ from six.moves import filter
 from six.moves.urllib.request import urlopen
 
 from galaxy import util
-from galaxy.util import (
-    compression_utils,
-    smart_str
-)
+from galaxy.util import compression_utils
 from galaxy.util.checkers import (
     check_binary,
     check_bz2,
@@ -63,33 +60,16 @@ def stream_to_open_named_file(stream, fd, filename, source_encoding=None, source
     """Writes a stream to the provided file descriptor, returns the file name. Closes file descriptor"""
     # signature and behavor is somewhat odd, due to backwards compatibility, but this can/should be done better
     CHUNK_SIZE = 1048576
-    data_checked = False
-    is_compressed = False
-    is_binary = False
     try:
         codecs.lookup(target_encoding)
     except Exception:
         target_encoding = util.DEFAULT_ENCODING  # utf-8
-    if not source_encoding:
-        source_encoding = util.DEFAULT_ENCODING  # sys.getdefaultencoding() would mimic old behavior (defaults to ascii)
     while True:
         chunk = stream.read(CHUNK_SIZE)
         if not chunk:
             break
-        if not data_checked:
-            # See if we're uploading a compressed file
-            try:
-                # Convert chunk to a bytestring if it is not already.
-                # Check if the first 2 bytes of the chunk are equal to the
-                # gzip magic number.
-                if smart_str(chunk)[:2] == util.gzip_magic:
-                    is_compressed = True
-            except Exception:
-                pass
-            if not is_compressed:
-                is_binary = util.is_binary(chunk)
-            data_checked = True
-        if not is_compressed and not is_binary:
+        if source_encoding is not None:
+            # If a source encoding is given we use it to convert to the target encoding
             if not isinstance(chunk, text_type):
                 chunk = chunk.decode(source_encoding, source_error)
             os.write(fd, chunk.encode(target_encoding, target_error))