Fix list item attachments

[ltdu]

Q	A
Bug fix?	[x]
New feature?	[ ]
New sample?	[ ]
Related issues?	fixes #282

This fixes #282 by changing how attachment files and previews are loaded. Additionally, it improves attachment related methods in SPService.

There are couple of things to keep in mind:

• because this changes public API it may require major version increase. SPService.getListItemAttachments() used to return null if no attachments there found. Now it returns []. But this is up to debate as I don't know whether anyone actually uses them directly.
• I'm pretty sure check for existing attachment didn't work before. Now it does and this control replaces existing file with new version. This may be unexpected to some users. Or not...
• I don't use this control in any of my projects so if messed up something let me know :)

[codecov-io]

Codecov Report

Merging #286 into dev will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev     #286   +/-   ##
=======================================
  Coverage   72.36%   72.36%           
=======================================
  Files          17       17           
  Lines         760      760           
  Branches      157      157           
=======================================
  Hits          550      550           
  Misses        163      163           
  Partials       47       47

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[estruyf]

Thanks @ltdu - this will be released as a bug fix. As you mention that there wasn't working something before, it might be that there wasn't a lot of usages yet + we will bump the major version soon as we plan to update to Office UI Fabric version 6 + SPFx 1.8.2 or higher.