iOS原生照片库视频格式为.mov，内存占用大，在发送文件中对它进行压缩。

- (void)movFileTransformToMP4WithSourcePath:(NSString *)sourcePath completion:(void(^)(NSString *Mp4FilePath))comepleteBlock session:(void(^)(AVAssetExportSession *session))sessionBlock
{
    /**
     *  mov格式转mp4格式
     */
    NSURL *sourceUrl = [NSURL URLWithString:sourcePath];
    
    AVURLAsset *avAsset = [AVURLAsset URLAssetWithURL:sourceUrl options:nil];
    
    NSArray *compatiblePresets = [AVAssetExportSession exportPresetsCompatibleWithAsset:avAsset];
    
    if ([compatiblePresets containsObject:AVAssetExportPresetMediumQuality]) {
        
        exportSession = [[AVAssetExportSession alloc] initWithAsset:avAsset presetName:AVAssetExportPresetMediumQuality];
        NSString *fileStr = [[sourcePath componentsSeparatedByString:@"/"].lastObject.uppercaseString stringByReplacingPercentEscapesUsingEncoding:NSUTF8StringEncoding];
        NSString *fileName = [[fileStr componentsSeparatedByString:@"."].firstObject.uppercaseString stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding];
        NSString *uniqueName = [NSString stringWithFormat:@"%@.mp4",fileName];
        NSArray *docPaths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
        NSString *docPath = docPaths.lastObject;
        NSString * resultPath = [docPath stringByAppendingPathComponent:uniqueName];
        
        exportSession.outputURL = [NSURL fileURLWithPath:resultPath];
        exportSession.outputFileType = AVFileTypeMPEG4;
        exportSession.shouldOptimizeForNetworkUse = YES;
        
        //如有此文件则直接返回
        if ([[NSFileManager defaultManager] fileExistsAtPath:resultPath]) {
            comepleteBlock(resultPath);
            return;
        }
        
        [exportSession exportAsynchronouslyWithCompletionHandler:^(void)
         {
             switch (exportSession.status) {
                     
                 case AVAssetExportSessionStatusUnknown:
                 {
                     NSLog(@"视频格式转换出错Unknown");
                     sessionBlock(exportSession);
                 }
                     break;
                     
                 case AVAssetExportSessionStatusWaiting:
                 {
                     NSLog(@"视频格式转换出错Waiting");
                     sessionBlock(exportSession);
                 }
                     break;
                     
                 case AVAssetExportSessionStatusExporting:
                 {
                     NSLog(@"视频格式转换出错Exporting");
                     sessionBlock(exportSession);
                 }
                     break;
                     
                 case AVAssetExportSessionStatusCompleted:
                 {
                     comepleteBlock(resultPath);
                     NSLog(@"mp4 file size:%lf MB",[NSData dataWithContentsOfURL:exportSession.outputURL].length/1024.f/1024.f);
                     NSData *da = [NSData dataWithContentsOfFile:resultPath];
                     NSLog(@"da:%lu",(unsigned long)da.length);
                 }
                     break;
                     
                 case AVAssetExportSessionStatusFailed:
                 {
                     NSLog(@"视频格式转换出错Unknown");
                     sessionBlock(exportSession);
                 }
                     break;
                     
                 case AVAssetExportSessionStatusCancelled:
                 {
                     NSLog(@"视频格式转换出错Cancelled");
                     sessionBlock(exportSession);
                 }
                     break;
             }
             
         }];
        
    }
}