CSV problems revealed by new cloudooo

New cloudooo uses text/csv as mimetype for CSV, which is correct, but this revealed issues in ERP5:
 - ERP5 did not detect text/csv for csv extensions
 - downloading a file with content type text/csv caused an error

See merge request !1859
16 jobs for feat/merge_a_bit_of_zope4py3 in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-Master

01:11:02

failed ERP5.UnitTest-Master

02:30:40

failed ERP5.UnitTest-Zope2

02:21:47

passed SlapOS.Eggs.UnitTest-Master.Python2

00:16:45

passed SlapOS.Eggs.UnitTest-Master.Python3

00:17:00

failed Wendelin.UnitTest-Master

01:21:28

passed ERP5.CodingStyleTest-Master

01:41:58

failed ERP5.UnitTest-Master

03:05:57

failed ERP5.UnitTest-Zope2

03:02:09

passed SlapOS.Eggs.UnitTest-Master.Python2

00:16:46

passed SlapOS.Eggs.UnitTest-Master.Python2

00:24:16

passed SlapOS.Eggs.UnitTest-Master.Python2

00:16:03

passed SlapOS.Eggs.UnitTest-Master.Python3

00:16:50

passed SlapOS.Eggs.UnitTest-Master.Python3

00:18:47

passed SlapOS.Eggs.UnitTest-Master.Python3

00:23:10

failed Wendelin.UnitTest-Master

01:28:47