I ran into this same issue as well.
Doing some research on the Android browser, the browser does not determine the file type from the file extension, like most desktop browsers. The Android browser determines the filetype through the ContentType property on the Response. To circumvent this, I just created a static dictionary initialized in Global.asax that sets mime types by file extension, like so....
public static void SetUpDictionary()
{
_FileExtentionToMimeTypeDictionary = new Dictionary<string, string>(33);
_FileExtentionToMimeTypeDictionary.Add("doc", "application/msword");
_FileExtentionToMimeTypeDictionary.Add("docx", "application/vnd.openxmlformats-officedocument.wordprocessingml.document");
_FileExtentionToMimeTypeDictionary.Add("jpeg", "image/jpeg");
_FileExtentionToMimeTypeDictionary.Add("jpg", "image/jpeg");
_FileExtentionToMimeTypeDictionary.Add("mpeg", "video/mpeg");
_FileExtentionToMimeTypeDictionary.Add("mpg", "video/mpeg");
_FileExtentionToMimeTypeDictionary.Add("pdf", "application/pdf");
_FileExtentionToMimeTypeDictionary.Add("zip", "application/zip");
_FileExtentionToMimeTypeDictionary.Add("xls", "application/excel");
_FileExtentionToMimeTypeDictionary.Add("xlw", "application/excel");
_FileExtentionToMimeTypeDictionary.Add("xlm", "application/excel");
_FileExtentionToMimeTypeDictionary.Add("xll", "application/excel");
_FileExtentionToMimeTypeDictionary.Add("xlt", "application/excel");
_FileExtentionToMimeTypeDictionary.Add("xlsx", "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet");
_FileExtentionToMimeTypeDictionary.Add("xlsb", "application/vnd.ms-excel.sheet.binary.macroEnabled.12");
_FileExtentionToMimeTypeDictionary.Add("tiff", "image/tiff");
_FileExtentionToMimeTypeDictionary.Add("tif", "image/tiff");
_FileExtentionToMimeTypeDictionary.Add("gif", "image/gif");
_FileExtentionToMimeTypeDictionary.Add("flv", "video/x-flv");
_FileExtentionToMimeTypeDictionary.Add("pot", "application/powerpoint");
_FileExtentionToMimeTypeDictionary.Add("ppt", "application/powerpoint");
_FileExtentionToMimeTypeDictionary.Add("pptx", "application/vnd.openxmlformats-officedocument.presentationml.presentation");
_FileExtentionToMimeTypeDictionary.Add("bmp", "image/bmp");
_FileExtentionToMimeTypeDictionary.Add("html", "text/html");
_FileExtentionToMimeTypeDictionary.Add("htm", "text/html");
_FileExtentionToMimeTypeDictionary.Add("htmls", "text/html");
_FileExtentionToMimeTypeDictionary.Add("rt", "text/richtext");
_FileExtentionToMimeTypeDictionary.Add("rtf", "text/richtext");
_FileExtentionToMimeTypeDictionary.Add("rtx", "text/richtext");
_FileExtentionToMimeTypeDictionary.Add("txt", "text/plain");
_FileExtentionToMimeTypeDictionary.Add("text", "text/plain");
_FileExtentionToMimeTypeDictionary.Add("xml", "text/xml");
}
Our company took those as the most uploaded file types, but there are hundreds more. By changing your ContentType, hopefully you'll get some different results.