Less knowledge so far ... as I expected.
Now I tried to create a new feature class and added a Blob field where I want to save my attachments. Also I added a Raster field for testing.
Then I tried on the client to save data to this fields. In the Raster field I could not save binary data, only bitmap. So, photos can be saved.
But - where is my Blob field? If I look my layer's table, the Blob field is missing!? No idea why.
I added two Blob fields for testing, both were not present on mobile client. Could Mobile 10 not work with Blob?
FileStream fs = new FileStream(@"C:\temp\MapCaches\Mobile10Tests\zeitplan.pdf", FileMode.OpenOrCreate, FileAccess.Read);
byte data = new byte[fs.Length];
fs.Read(data, 0, Convert.ToInt32(fs.Length));
Bitmap bitmap = new Bitmap(@"C:\temp\MapCaches\Mobile10Tests\bild.jpg");
var table = ((FeatureLayer) mobileService2.Layers["Attachments"]).GetDataTable();
var row = table.NewRow();
row["Raster1"] = bitmap; //-> works
row["Binary1"] = data; //-> no Binary1 field found!