Probably not natively supported by Apple. But I'm guessing that the Files app just builds on the File Provider extension mechanism [1] that provides the document picker in apps, so Transmit can probably expose your servers too.
[1] https://developer.apple.com/library/content/documentation/Ge...
Re:iPad Pro
by walterbell on Tuesday June 06, 2017 @03:37AM
(#14495441)
Yes, Transmit can add attachments today.
Hopefully they will be able to add the same iOS11 drag-and-drop features as Apple Files.
Currently, Transmit for iOS does file management, including attaching files to emails, https://panic.com/transmit-ios/