Issue #211: ingest/importers/regexp.py tests failing (blais/beancount)

[droogmic]

New issue 211: ingest/importers/regexp.py tests failing
https://bitbucket.org/blais/beancount/issues/211/ingest-importers-regexppy-tests-failing

droogmic:

```
#!python
FAIL: test_match (regexp_test.TestRegexpMixin)
----------------------------------------------------------------------
Traceback (most recent call last):
File ".../beancount/utils/test_utils.py", line 224, in new_function
return function(self, file.name)
File ".../beancount/ingest/importers/regexp_test.py", line 45, in test_match
self.assertTrue(importer.identify(file))
AssertionError: False is not true
```

Cause:
The tempfile is detected as "MimeType: text/x-Algol68", when the tests expect "MimeType: text/plain"

e.g.
```
#!python
>>> print(match_text)
"""
Filename: /tmp/tmptr9cn666
MimeType: text/x-Algol68
Contents:
DATE,TYPE,REF #,DESCRIPTION,FEES,AMOUNT,BALANCE
2014-04-14,BUY,14167001,BOUGHT +CSKO 50 @98.35,7.95,-4925.45,25674.63
2014-05-08,BUY,12040838,BOUGHT +HOOL 121 @79.11,7.95,-9580.26,16094.37
2014-05-11,BUY,41579908,BOUGHT +MSFX 104 @64.39,7.95,-6704.51,9389.86
2014-05-22,DIV,54857517,ORDINARY DIVIDEND~HOOL,0,28.56,9418.42
"""
```