Sharing a file to a user directly will result in the file not making it through to the input field if the W-A activity has previously been dismissed.
Reproduction steps:
Open W-A and ensure it is connected.
Open the task switcher and dismiss/slide W-A out of view so that it is no longer present.
Open the system's file manager or gallery.
Select a file and tap share.
Ensure that the share dialog is the one that lets you pick a user/contact to directly share to, and select someone. If it's not, try a different app to share from. Sharing to W-A itself will not cause this issue.
W-A opens but the file hasn't made it through to the input field.
Sharing a file to a user directly will result in the file not making it through to the input field if the W-A activity has previously been dismissed.
Reproduction steps: