Edited:
Use the following method to retrieve the MIME type of an image from the gallery:
public static String GetMimeType(Context context, Uri uriImage)
{
    String strMimeType = null;
    Cursor cursor = context.getContentResolver().query(uriImage,
                        new String[] { MediaStore.MediaColumns.MIME_TYPE },
                        null, null, null);
    if (cursor != null && cursor.moveToNext())
    {
        strMimeType = cursor.getString(0);
    }
    return strMimeType;
}
This will return something like "image/jpeg".
Previous answer:
You can use the following code to convert the image from the Gallery to the format you want, like a JPG:
ByteArrayOutputStream outputBuffer = new ByteArrayOutputStream();
Bitmap bitmapImage = BitmapFactory.decodeStream(
                         getContentResolver().openInputStream(myImageUri));
if (bitmapImage.compress(Bitmap.CompressFormat.JPEG, 100, outputBuffer))
{
    // Then perform a base64 of the byte array...
}
This way you will control the image format your are sending to the server, and can even compress more to save bandwidth. ;)