Allow using existing files

• Add helper method to get form ID for given file ID
• Allow attachPdfToForm method to be re-used for permanent files

added 4 commits

• fa01a4e9...92cfe96a - 2 commits from branch project:5.2.x
• 689715c7 - Add helper method to get form ID for given file ID
• 458826ab - Allow attachPdfToForm method to be re-used for permanent files

Compare with previous version

added 2 commits

• e8a40721 - Allow simpler D7-style JSON (from issue 3502991)
• 55486af5 - Add file field formatter for linking to file in fillpdf's populate PDF route

Compare with previous version

added 1 commit

• 4d217830 - Add a field formatter to link to FillPDF edit page for paired media

Compare with previous version