Given the files:

• test.pdf (signed file)
• detached.p7s (detached smime signature)

Currently the only way to verify detached.p7s is to rename it to test.pdf.p7s.:

• When you open detached.p7s (both via explorer or decrypt/verify file), the file cannot be chosen, and a bad signature is reported (probably against the not existing file "detached"):
  

• If you try to decrypt/verify test.pdf, you can only choose the signed file, not the signature file:
  

If possible, the same workflow as for pgp sig files should be applied:
If the signed file does not exist, display a "select file" dialog to be able to manually select the signed file:

Note: The screenshots are from vsd-3.4.0-beta1344 @ win10 but the same applies to vsd-4.0.0-beta1330 @ win10