Yes, for code completion, Textastic supports snippets including tab stops, placeholders, mirrors and transformations as described at http://manual.macromates.com/en/snippets

Textastic does support transformations in snippets, but I don't know if that this is what you are referring to.

See https://github.com/blach/Textastic-Customization#templates

Have a look at the TextMate manual entry on snippets for an overview of the snippet syntax. Textastic supports tab stops, placeholders, mirrors and transformations.

If this is not what you mean: Can you give me more details? What do these transformations do?

No, sorry. I don't use jailbreaks myself.

This seems to be the same problem as with .c files and is most likely a bug in Google Drive.

I can confirm that I can not open .c files from Google Drive.

It looks like Google Drive is not correctly implementing this, since it does work when I try to open a .c file from iCloud Drive. So it would probably best to report this bug to Google.