It is not possible to preview attachments in the Proton Mail Android app without them being downloaded to the "Downloads" folder of the device, which unneccessarily clutters the folder with content you only meant to view and maybe then decide to download.

The Proton Drive app does let you preview files without them ending up on the visible device storage, so this should be possible.

Related, but really not a real preview feature, at least not on Android:
https://protonmail.uservoice.com/forums/284483-proton-mail/suggestions/7808634-preview-attachments