There are a number of common document types in use that the mobile version of Proton Drive doesn't support. One of these is TXT format. Please add support to read basic text encodings ASCII/ANSI/Unicode/UTF-8? I might have missed some since this is off the top of my head.

It seems like there quite a number of pretty common document formats that are not readable from the mobile version of Proton Drive or Proton Docs. Examples:
RTF/HTML - https://protonmail.uservoice.com/forums/932839-proton-drive/suggestions/50648297-paste-markdown-into-docs
Markdown - https://protonmail.uservoice.com/forums/932839-proton-drive/suggestions/50095551-markdown-support-for-drive