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.

Unfortunately, it is not possible to add a file type at runtime. All supported file types are defined in an "Info.plist" file which is static and must be defined by the app developer at compile time of the app.

I am currently working on it.

Settings -> General -> Language & Region -> Preferred Language Order (use the Edit button in the top right corner to change the language order)

When you use the "Open…" command in Textastic, the operating system opens Textastic's sandbox so that it can access the file you choose in the document picker. Textastic does not have access to other files in the same folder, so the web preview screen does not show CSS or image files.

You could also use the Preview feature in Working Copy instead. It trivially has access to all the files and can include everything in its preview.

When Textastic invokes the document picker, it limits the types of files to the following system-declared uniform type identifiers: "public.text", "public.plain-text", "public.source-code" and "public.script". This is done because it cannot edit files such as image or video files.

The ".jl" file extension is by default not known to the system so there is no uniform type identifier declared for it. This means that document providers typically do not allow to open such a file.