Fix #422 "fieldUpload event throws ReferenceError in Firefox"

[ik9999]

• Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
  Bug fix

• What is the current behavior? (You can also link to an open issue here)
  Firefox throws reference error when using fieldUpload and onChange callback

• What is the new behavior (if this is a feature change)?
  Firefox doesnt throw reference error and user get the event object in onChange callback function

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)
  No

• Other information:

[zoul0813]

can you change event to $event - as event is a reserved word in most runtimes.

[ik9999]

@zoul0813 Done.

[zoul0813]

Thanks for the fix @ik9999