Loading image from non-bundle file

In my app, we're creating an image using a Signature Pad, and then assigning the result to an Image

tempImage1.Source = new FileImageSource { File = sig.Filename };

this works great on iOS, with an image path like this

/Users/jason/Library/Developer/CoreSimulator/Devices/54F766C3-B64E-4E2C-99B2-F0363C1A780D/data/Containers/Data/Application/70CD9F58-898B-45E4-B7F3-F2C4E4B54CDA/tmp/tmp5774fbbe.png

however, Android does NOT display the image (using this path)

/data/data/m.transport.Android.Alpha/cache/tmp28150193.png

in both cases FileImageSource is created successfully without any errors or exceptions

Has anyone else been able to do this? Is there a reason why this wouldn't work on Android?