Please recognize that MobileSheets for Android is an Android application, first and foremost, and is not a ChromeOS application. It just happens to run on Chromebooks due to the fact that Google emulates and has support for Android applications. So I'm not going to try to tailor the experience for Chromebook devices. The handling of external storage (that is what "My Files" is from the perspective of the Android SDK) accounts for SD cards, USB thumb drives, etc, and so the UIs that you see are because the "My Files" is being handled like a removable SD card. That is because it appears as the same type of storage as a removable SD card, hence why the instructional dialog calls out SD cards.
If you want to import from "My Files", the best way to do that is Import->System File Browser, which uses Google's file picker.
I should say that, on my Google Pixel Slate, Import->Local File acts EXACTLY like it does on an Android device if you don't tap the SD card icon and try to jump to other locations. If you tap the SD card icon and then "Device", it takes you to /storage/emulated/0. You can then tap the Download folder, for example, and import anything you want under there. You could change the storage location to a folder under /storage/emulated/0 as well, and if you do that, then you'll have access to that from any other Android location that can access that folder. The ChromeOS file manager probably can't, because it's not designed as an Android application - it's a ChromeOS application. So you are going to run into issues if you try to mix the two. Instead, install an Android file manager like File Manager +, and that will be able to access the downloads folder (assuming it has the all files access permission, otherwise it will have to use Google's file picker or be limited in other ways).
I think you significantly complicated things by trying to involve the "My Files" section of the storage, most likely because you wanted to use the ChromeOS file manager (which is a completely natural thing to want to do). I just don't know what the best way is to inform users of the potential pitfalls of using Android applications on ChromeOS, because this is not a MobileSheets specific issue.
Mike
If you want to import from "My Files", the best way to do that is Import->System File Browser, which uses Google's file picker.
I should say that, on my Google Pixel Slate, Import->Local File acts EXACTLY like it does on an Android device if you don't tap the SD card icon and try to jump to other locations. If you tap the SD card icon and then "Device", it takes you to /storage/emulated/0. You can then tap the Download folder, for example, and import anything you want under there. You could change the storage location to a folder under /storage/emulated/0 as well, and if you do that, then you'll have access to that from any other Android location that can access that folder. The ChromeOS file manager probably can't, because it's not designed as an Android application - it's a ChromeOS application. So you are going to run into issues if you try to mix the two. Instead, install an Android file manager like File Manager +, and that will be able to access the downloads folder (assuming it has the all files access permission, otherwise it will have to use Google's file picker or be limited in other ways).
I think you significantly complicated things by trying to involve the "My Files" section of the storage, most likely because you wanted to use the ChromeOS file manager (which is a completely natural thing to want to do). I just don't know what the best way is to inform users of the potential pitfalls of using Android applications on ChromeOS, because this is not a MobileSheets specific issue.
Mike