could you post the complete list of those file extensions not working under "Force download" option using mobile Chrome on Android from your experience?
Moodle already manages many MIME types e.g. docx (https://github.com/moodle/moodle/blob/9528f643d1dd02896bc8326ae63b4220d55780e9/lib/filelib.php#L1418) and "Force download" changes the content disposition header from inline to attachment but there are issues on Android Chrome e.g. https://code.google.com/p/android/issues/detail?id=11422 or other telling about the file extension to be in upper case if you want to download it in old Android versions.
Having the full list of those extensions will help others reading this thread first and maybe Devs will try to understand if Moodle could do something to mitigate this Android Chrome issue.
For example, here is my first proposal: you could try to change the line at https://github.com/moodle/moodle/blob/9528f643d1dd02896bc8326ae63b4220d55780e9/lib/filelib.php#L2265 forcing $isFF always to true to see if sending the right MIME type and not the fake application/x-forcedownload will solve your issue.
Waiting for your feedback, I'll try to understand the historical reasons for that Moodle check since IMHO it is not correct regardless, potentially, only mobile Chrome complains about it: indeed, if the aim is to force HTML files download it should be limited to that case. This is IMHO a bug and, if confirmed from you, I'll be happy to open an issue into the Tracker and propose a fix for it.