Fix template variable type checking

[Blackbaud-SteveBrush]

Angular doesn't like it when template variables' types are ambiguous. Pulling the variables into the class fixes it.

[Blackbaud-BobbyEarl]

Preview build at https://host.nxt.blackbaud.com/skyux-forms?_sp.spa=pr-fix-template-variables

[codecov]

Codecov Report

Merging #100 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #100   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          25     25           
  Lines         907    913    +6     
  Branches      128    128           
=====================================
+ Hits          907    913    +6

Impacted Files	Coverage Δ
...lic/modules/file-attachment/file-item.component.ts	100% <100%> (ø)	⬆️

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