Support "Drag and drop file upload to file type custom field"

[sanak]

Problem
During #24 work, I noticed that the following changes are missing in gtt/develop branch's override.

• redmine.org issue: Feature #30776: Drag and drop file upload to file type custom field - Redmine
• diff: https://redmine.org/projects/redmine/repository/svn/revisions/20712/diff

Expectation
This plugin's override should keep upstream functionality.
Probably, dropping Redmine < 4.2 will be necessary for supporting this.

[sanak]

This needs to be checked for Redmine 5.1.