Fix UnicodeDecodeError on Attachment upload

[ramonski]

Description of the issue/feature this PR addresses

This PR fixes a UnicodeDecodeError that happens when an attachment is assigned to an Analysis with a title containing unicode characters.

Traceback:

Traceback (innermost last):
  Module ZServer.ZPublisher.Publish, line 144, in publish
  Module ZPublisher.mapply, line 85, in mapply
  Module ZServer.ZPublisher.Publish, line 44, in call_object
  Module senaite.core.browser.attachment.attachment, line 91, in __call__
  Module senaite.core.browser.attachment.attachment, line 266, in action_add
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 10: ordinal not in range(128)

Current behavior before PR

Traceback occurs when an attachment is uploaded in a sample/worksheet and assigned to an analysis with a title containing unicode characters.

Desired behavior after PR is merged

Upload works for all analysis titles.

--
I confirm I have tested this PR thoroughly and coded it according to PEP8
and Plone's Python styleguide standards.

[xispa]

Argg, this is always a brain killer!