support sending all files

use generic mimetype when file has unknown extension

// FREEBIE

Signal/src/ViewControllers/SignalAttachment.swift

@@ -12,7 +12,6 @@ enum SignalAttachmentError: Error {
     case couldNotParseImage
     case couldNotConvertToJpeg
     case invalidFileFormat
-    case unknownType
 }
 
 extension SignalAttachmentError: LocalizedError {
@@ -30,8 +29,6 @@ extension SignalAttachmentError: LocalizedError {
             return NSLocalizedString("ATTACHMENT_ERROR_COULD_NOT_CONVERT_TO_JPEG", comment: "Attachment error message for image attachments which could not be converted to JPEG")
         case .invalidFileFormat:
             return NSLocalizedString("ATTACHMENT_ERROR_INVALID_FILE_FORMAT", comment: "Attachment error message for attachments with an invalid file format")
-        case .unknownType:
-            return NSLocalizedString("ATTACHMENT_ERROR_UNKNOWN_TYPE", comment: "Attachment error message for attachments with an invalid file format")
         }
     }
 }
@@ -114,10 +111,6 @@ class SignalAttachment: NSObject {
         self.dataUTI = dataUTI
         self.filename = filename
         super.init()
-
-        if self.mimeType == nil {
-            error = .unknownType
-        }
     }
 
     // MARK: Methods
@@ -174,7 +167,7 @@ class SignalAttachment: NSObject {
 
     // Returns the MIME type for this attachment or nil if no MIME type
     // can be identified.
-    var mimeType: String? {
+    var mimeType: String {
         if dataUTI == SignalAttachment.kOversizeTextAttachmentUTI {
             return OWSMimeTypeOversizeTextMessage
         }
@@ -182,7 +175,7 @@ class SignalAttachment: NSObject {
             return OWSMimeTypeUnknownForTests
         }
         guard let mimeType = UTTypeCopyPreferredTagWithClass(dataUTI as CFString, kUTTagClassMIMEType) else {
-            return nil
+            return OWSMimeTypeApplicationOctetStream
         }
         return mimeType.takeRetainedValue() as String
     }